Gradients not working well with 16bit color depth

Merry Xmas!
I´m having troubles with the way that gradients are displayed when color depth is setted to 16. In fact ther is no gradient but a group of 5 or 6 colors. You can see the effect in the capture of the simulator: 32 left, 16 right

I´m using a ESP32S3 with a 5" buydisplay ER-TFTM050A2-3-3661 800x480 8 bit 8080 parallel interface

8.1 LVGL version

I tried 32 & 16 bits in the sumilator with the same results ith 16 bit configuration. I can´t use 32 in the device due to hardware limitations

Here my LVLG object

// RESET STYLE
lv_style_t resetStyle;
lv_style_init(&_resetStyle);
lv_style_set_border_width(&_resetStyle, 0);
lv_style_set_bg_opa(&_resetStyle, LV_OPA_TRANSP);
lv_style_set_pad_all(&_resetStyle, 0);

// MAIN GRADIENT STYLE
lv_style_t _mainGradient;
lv_style_init(&_mainGradient);
lv_style_set_bg_opa(&_mainGradient, LV_OPA_COVER);
lv_style_set_bg_color(&_mainGradient, lv_color_hex(0x2B2E2D));
lv_style_set_bg_grad_color(&_mainGradient, lv_color_hex(0X1A1D1C));
lv_style_set_bg_grad_dir(&_mainGradient, LV_GRAD_DIR_VER);

// DIAL SHADOW STYLE
lv_style_t _dialShadow;
lv_style_init(&_dialShadow);
lv_style_set_shadow_width(&_dialShadow, 55);
lv_style_set_shadow_color(&_dialShadow, lv_color_hex(0x2D2D2D));
lv_style_set_shadow_ofs_y(&_dialShadow, 7);

//DIAL BACKGROUND
lv_obj_t *circle = lv_obj_create(top);
lv_obj_set_size(circle, 372, 372);
lv_obj_align(circle, LV_ALIGN_TOP_MID, 0, 45);
lv_obj_add_style(circle, CitylStyle::getWrapperStyle(), 0);
lv_obj_add_style(circle, CitylStyle::getMainGradientStyle(), 0);
lv_obj_add_style(circle, CitylStyle::getDialShadowStyle(), 0);
lv_obj_set_style_radius(circle, LV_RADIUS_CIRCLE, 0);
lv_obj_add_flag(circle, LV_OBJ_FLAG_CLICKABLE);

// DIAL
lv_obj_t * arc = lv_arc_create(circle);
lv_obj_set_size(arc, 308, 308);
lv_arc_set_rotation(arc, 270);
lv_arc_set_bg_angles(arc, 0, 360);

Here my lv_conf.h

#pragma once

#include <stdint.h>

#if defined(LINUX_OS) || defined(WIN_OS)
	#define LV_HOR_RES_MAX          480
	#define LV_VER_RES_MAX          800
#endif

/*====================
   COLOR SETTINGS
 *====================*/

#if defined(LINUX_OS) || defined(WIN_OS)
  #define LV_COLOR_DEPTH     32
  /* Swap the 2 bytes of RGB565 color.
  * Useful if the display has a 8 bit interface (e.g. SPI)*/
  #define LV_COLOR_16_SWAP   0
#else
  #define LV_COLOR_DEPTH     16
  /* Swap the 2 bytes of RGB565 color.
  * Useful if the display has a 8 bit interface (e.g. SPI)*/
  #define LV_COLOR_16_SWAP   1
#endif

/*Enable more complex drawing routines to manage screens transparency.
 *Can be used if the UI is above another layer, e.g. an OSD menu or video player.
 *Requires `LV_COLOR_DEPTH = 32` colors and the screen's `bg_opa` should be set to non LV_OPA_COVER value*/
#define LV_COLOR_SCREEN_TRANSP 0

/* Adjust color mix functions rounding. GPUs might calculate color mix (blending) differently.
 * 0: round down, 64: round up from x.75, 128: round up from half, 192: round up from x.25, 254: round up */
#define LV_COLOR_MIX_ROUND_OFS (LV_COLOR_DEPTH == 32 ? 0: 128)

/*Images pixels with this color will not be drawn if they are chroma keyed)*/
#define LV_COLOR_CHROMA_KEY lv_color_hex(0x00ff00)         /*pure green*/

/*=========================
   MEMORY SETTINGS
 *=========================*/

/*1: use custom malloc/free, 0: use the built-in `lv_mem_alloc()` and `lv_mem_free()`*/
#define LV_MEM_CUSTOM 1
#if LV_MEM_CUSTOM == 0
    /*Size of the memory available for `lv_mem_alloc()` in bytes (>= 2kB)*/
    #define LV_MEM_SIZE (32U * 1024U)          /*[bytes]*/

    /*Set an address for the memory pool instead of allocating it as a normal array. Can be in external SRAM too.*/
    #define LV_MEM_ADR 0     /*0: unused*/
    /*Instead of an address give a memory allocator that will be called to get a memory pool for LVGL. E.g. my_malloc*/
    #if LV_MEM_ADR == 0
        //#define LV_MEM_POOL_INCLUDE your_alloc_library  /* Uncomment if using an external allocator*/
        //#define LV_MEM_POOL_ALLOC   your_alloc          /* Uncomment if using an external allocator*/
    #endif

#else       /*LV_MEM_CUSTOM*/
    #if defined(LINUX_OS) || defined(WIN_OS)
    #  define LV_MEM_CUSTOM_INCLUDE "stdlib.h"   /*Header for the dynamic memory function*/
    #  define LV_MEM_CUSTOM_ALLOC   malloc       /*Wrapper to malloc*/
    #  define LV_MEM_CUSTOM_FREE    free         /*Wrapper to free*/
    #  define LV_MEM_CUSTOM_REALLOC realloc
    #else
    #  define LV_MEM_CUSTOM_INCLUDE "my_global.h"   /*Header for the dynamic memory function*/
    #  define LV_MEM_CUSTOM_ALLOC   bsp_malloc      /*Wrapper to malloc -> ps_malloc to use external SPI RAM */
    #  define LV_MEM_CUSTOM_FREE    bsp_free         /*Wrapper to free*/
    #  define LV_MEM_CUSTOM_REALLOC bsp_realloc
    #endif    
#endif     /*LV_MEM_CUSTOM*/

/*Number of the intermediate memory buffer used during rendering and other internal processing mechanisms.
 *You will see an error log message if there wasn't enough buffers. */
#define LV_MEM_BUF_MAX_NUM 16

/*Use the standard `memcpy` and `memset` instead of LVGL's own functions. (Might or might not be faster).*/
#define LV_MEMCPY_MEMSET_STD 0

/*====================
   HAL SETTINGS
 *====================*/

/*Default display refresh period. LVG will redraw changed areas with this period time*/
#define LV_DISP_DEF_REFR_PERIOD 30      /*[ms]*/

/*Input device read period in milliseconds*/
#define LV_INDEV_DEF_READ_PERIOD 30     /*[ms]*/

/*Use a custom tick source that tells the elapsed time in milliseconds.
 *It removes the need to manually update the tick with `lv_tick_inc()`)*/
#define LV_TICK_CUSTOM 0
#if LV_TICK_CUSTOM
    #define LV_TICK_CUSTOM_INCLUDE "Arduino.h"         /*Header for the system time function*/
    #define LV_TICK_CUSTOM_SYS_TIME_EXPR (millis())    /*Expression evaluating to current system time in ms*/
#endif   /*LV_TICK_CUSTOM*/

/*Default Dot Per Inch. Used to initialize default sizes such as widgets sized, style paddings.
 *(Not so important, you can adjust it to modify default sizes and spaces)*/
#define LV_DPI_DEF 130     /*[px/inch]*/

/*=======================
 * FEATURE CONFIGURATION
 *=======================*/

/*-------------
 * Drawing
 *-----------*/

/*Enable complex draw engine.
 *Required to draw shadow, gradient, rounded corners, circles, arc, skew lines, image transformations or any masks*/
#define LV_DRAW_COMPLEX 1
#if LV_DRAW_COMPLEX != 0

    /*Allow buffering some shadow calculation.
    *LV_SHADOW_CACHE_SIZE is the max. shadow size to buffer, where shadow size is `shadow_width + radius`
    *Caching has LV_SHADOW_CACHE_SIZE^2 RAM cost*/
    #define LV_SHADOW_CACHE_SIZE 0

    /* Set number of maximally cached circle data.
    * The circumference of 1/4 circle are saved for anti-aliasing
    * radius * 4 bytes are used per circle (the most often used radiuses are saved)
    * 0: to disable caching */
    #define LV_CIRCLE_CACHE_SIZE 4

#endif /*LV_DRAW_COMPLEX*/

/*Default image cache size. Image caching keeps the images opened.
 *If only the built-in image formats are used there is no real advantage of caching. (I.e. if no new image decoder is added)
 *With complex image decoders (e.g. PNG or JPG) caching can save the continuous open/decode of images.
 *However the opened images might consume additional RAM.
 *0: to disable caching*/
#define LV_IMG_CACHE_DEF_SIZE 0

/*Maximum buffer size to allocate for rotation. Only used if software rotation is enabled in the display driver.*/
#define LV_DISP_ROT_MAX_BUF (10*1024)

/*-------------
 * GPU
 *-----------*/

/*Use STM32's DMA2D (aka Chrom Art) GPU*/
#define LV_USE_GPU_STM32_DMA2D 0
#if LV_USE_GPU_STM32_DMA2D
    /*Must be defined to include path of CMSIS header of target processor
    e.g. "stm32f769xx.h" or "stm32f429xx.h"*/
    #define LV_GPU_DMA2D_CMSIS_INCLUDE
#endif

/*Use NXP's PXP GPU iMX RTxxx platforms*/
#define LV_USE_GPU_NXP_PXP 0
#if LV_USE_GPU_NXP_PXP
    /*1: Add default bare metal and FreeRTOS interrupt handling routines for PXP (lv_gpu_nxp_pxp_osa.c)
    *   and call lv_gpu_nxp_pxp_init() automatically during lv_init(). Note that symbol SDK_OS_FREE_RTOS
    *   has to be defined in order to use FreeRTOS OSA, otherwise bare-metal implementation is selected.
    *0: lv_gpu_nxp_pxp_init() has to be called manually before lv_init()
    */
    #define LV_USE_GPU_NXP_PXP_AUTO_INIT 0
#endif

/*Use NXP's VG-Lite GPU iMX RTxxx platforms*/
#define LV_USE_GPU_NXP_VG_LITE 0

/*Use exnternal renderer*/
#define LV_USE_EXTERNAL_RENDERER 0

/*Use SDL renderer API. Requires LV_USE_EXTERNAL_RENDERER*/
#define LV_USE_GPU_SDL 0
#if LV_USE_GPU_SDL
    #define LV_GPU_SDL_INCLUDE_PATH <SDL2/SDL.h>
#endif

/*-------------
 * Logging
 *-----------*/

/*Enable the log module*/
#define LV_USE_LOG 1
#if LV_USE_LOG

    /*How important log should be added:
    *LV_LOG_LEVEL_TRACE       A lot of logs to give detailed information
    *LV_LOG_LEVEL_INFO        Log important events
    *LV_LOG_LEVEL_WARN        Log if something unwanted happened but didn't cause a problem
    *LV_LOG_LEVEL_ERROR       Only critical issue, when the system may fail
    *LV_LOG_LEVEL_USER        Only logs added by the user
    *LV_LOG_LEVEL_NONE        Do not log anything*/
    #define LV_LOG_LEVEL LV_LOG_LEVEL_TRACE

    /*1: Print the log with 'printf';
    *0: User need to register a callback with `lv_log_register_print_cb()`*/
    #define LV_LOG_PRINTF 0

    /*Enable/disable LV_LOG_TRACE in modules that produces a huge number of logs*/
    #define LV_LOG_TRACE_MEM        1
    #define LV_LOG_TRACE_TIMER      1
    #define LV_LOG_TRACE_INDEV      1
    #define LV_LOG_TRACE_DISP_REFR  1
    #define LV_LOG_TRACE_EVENT      1
    #define LV_LOG_TRACE_OBJ_CREATE 1
    #define LV_LOG_TRACE_LAYOUT     1
    #define LV_LOG_TRACE_ANIM       1

#endif  /*LV_USE_LOG*/

/*-------------
 * Asserts
 *-----------*/

/*Enable asserts if an operation is failed or an invalid data is found.
 *If LV_USE_LOG is enabled an error message will be printed on failure*/
#define LV_USE_ASSERT_NULL          1   /*Check if the parameter is NULL. (Very fast, recommended)*/
#define LV_USE_ASSERT_MALLOC        1   /*Checks is the memory is successfully allocated or no. (Very fast, recommended)*/
#define LV_USE_ASSERT_STYLE         0   /*Check if the styles are properly initialized. (Very fast, recommended)*/
#define LV_USE_ASSERT_MEM_INTEGRITY 0   /*Check the integrity of `lv_mem` after critical operations. (Slow)*/
#define LV_USE_ASSERT_OBJ           0   /*Check the object's type and existence (e.g. not deleted). (Slow)*/

/*Add a custom handler when assert happens e.g. to restart the MCU*/
#define LV_ASSERT_HANDLER_INCLUDE <stdint.h>
#define LV_ASSERT_HANDLER while(1);   /*Halt by default*/

/*-------------
 * Others
 *-----------*/

/*1: Show CPU usage and FPS count*/
#define LV_USE_PERF_MONITOR 0
#if LV_USE_PERF_MONITOR
    #define LV_USE_PERF_MONITOR_POS LV_ALIGN_BOTTOM_RIGHT
#endif

/*1: Show the used memory and the memory fragmentation
 * Requires LV_MEM_CUSTOM = 0*/
#define LV_USE_MEM_MONITOR 0
#if LV_USE_MEM_MONITOR
    #define LV_USE_MEM_MONITOR_POS LV_ALIGN_BOTTOM_LEFT
#endif

/*1: Draw random colored rectangles over the redrawn areas*/
#define LV_USE_REFR_DEBUG 0

/*Change the built in (v)snprintf functions*/
#define LV_SPRINTF_CUSTOM 0
#if LV_SPRINTF_CUSTOM
    #define LV_SPRINTF_INCLUDE <stdio.h>
    #define lv_snprintf  snprintf
    #define lv_vsnprintf vsnprintf
#else   /*LV_SPRINTF_CUSTOM*/
    #define LV_SPRINTF_USE_FLOAT 0
#endif  /*LV_SPRINTF_CUSTOM*/

#define LV_USE_USER_DATA 1

/*Garbage Collector settings
 *Used if lvgl is bound to higher level language and the memory is managed by that language*/
#define LV_ENABLE_GC 0
#if LV_ENABLE_GC != 0
    #define LV_GC_INCLUDE "gc.h"                           /*Include Garbage Collector related things*/
#endif /*LV_ENABLE_GC*/

/*=====================
 *  COMPILER SETTINGS
 *====================*/

/*For big endian systems set to 1*/
#define LV_BIG_ENDIAN_SYSTEM 0

/*Define a custom attribute to `lv_tick_inc` function*/
#define LV_ATTRIBUTE_TICK_INC

/*Define a custom attribute to `lv_timer_handler` function*/
#define LV_ATTRIBUTE_TIMER_HANDLER

/*Define a custom attribute to `lv_disp_flush_ready` function*/
#define LV_ATTRIBUTE_FLUSH_READY

/*Required alignment size for buffers*/
#define LV_ATTRIBUTE_MEM_ALIGN_SIZE 1

/*Will be added where memories needs to be aligned (with -Os data might not be aligned to boundary by default).
 * E.g. __attribute__((aligned(4)))*/
#define LV_ATTRIBUTE_MEM_ALIGN

/*Attribute to mark large constant arrays for example font's bitmaps*/
#define LV_ATTRIBUTE_LARGE_CONST

/*Complier prefix for a big array declaration in RAM*/
#define LV_ATTRIBUTE_LARGE_RAM_ARRAY

/*Place performance critical functions into a faster memory (e.g RAM)*/
#define LV_ATTRIBUTE_FAST_MEM

/*Prefix variables that are used in GPU accelerated operations, often these need to be placed in RAM sections that are DMA accessible*/
#define LV_ATTRIBUTE_DMA

/*Export integer constant to binding. This macro is used with constants in the form of LV_<CONST> that
 *should also appear on LVGL binding API such as Micropython.*/
#define LV_EXPORT_CONST_INT(int_value) struct _silence_gcc_warning /*The default value just prevents GCC warning*/

/*Extend the default -32k..32k coordinate range to -4M..4M by using int32_t for coordinates instead of int16_t*/
#define LV_USE_LARGE_COORD 0

/*==================
 *   FONT USAGE
 *===================*/

/* Montserrat fonts with bpp = 4
 * https://fonts.google.com/specimen/Montserrat  */
#if defined (WIN_OS) || defined(LINUX_OS)
#define LV_FONT_SANS_8     1
#define LV_FONT_SANS_12    1
#define LV_FONT_SANS_14    1
#define LV_FONT_SANS_16    1
#define LV_FONT_SANS_20    1
#define LV_FONT_SANS_22    1
#define LV_FONT_SANS_24    1
#define LV_FONT_SANS_28    1
#define LV_FONT_SANS_30    1
#define LV_FONT_SANS_34    1
#define LV_FONT_SANS_42    1
#define LV_FONT_SANS_48    1
#define LV_FONT_SANS_52    1
#define LV_FONT_SANS_54    1
#define LV_FONT_SANS_56    1
#define LV_FONT_SANS_60    1
#define LV_FONT_SANS_70    1
#else
#define LV_FONT_SANS_8     1
#define LV_FONT_SANS_12    1
#define LV_FONT_SANS_14    1
#define LV_FONT_SANS_16    1
#define LV_FONT_SANS_20    1
#define LV_FONT_SANS_22    1
#define LV_FONT_SANS_24    1
#define LV_FONT_SANS_28    1
#define LV_FONT_SANS_30    1
#define LV_FONT_SANS_34    1
#define LV_FONT_SANS_42    1
#define LV_FONT_SANS_48    1
#define LV_FONT_SANS_52    1
#define LV_FONT_SANS_54    1
#define LV_FONT_SANS_56    1
#define LV_FONT_SANS_60    1
#define LV_FONT_SANS_70    1
#endif

/*Demonstrate special features*/
#define LV_FONT_MONTSERRAT_12_SUBPX      0
#define LV_FONT_MONTSERRAT_28_COMPRESSED 0  /*bpp = 3*/
#define LV_FONT_DEJAVU_16_PERSIAN_HEBREW 0  /*Hebrew, Arabic, Perisan letters and all their forms*/
#define LV_FONT_SIMSUN_16_CJK            0  /*1000 most common CJK radicals*/

/*Pixel perfect monospace fonts*/
#define LV_FONT_UNSCII_8  0
#define LV_FONT_UNSCII_16 0

/*Optionally declare custom fonts here.
 *You can use these fonts as default font too and they will be available globally.
 *E.g. #define LV_FONT_CUSTOM_DECLARE   LV_FONT_DECLARE(my_font_1) LV_FONT_DECLARE(my_font_2)*/
#define LV_FONT_CUSTOM_DECLARE

/*Always set a default font*/
#define LV_FONT_DEFAULT &lv_font_montserrat_14

/*Enable handling large font and/or fonts with a lot of characters.
 *The limit depends on the font size, font face and bpp.
 *Compiler error will be triggered if a font needs it.*/
#define LV_FONT_FMT_TXT_LARGE 0

/*Enables/disables support for compressed fonts.*/
#define LV_USE_FONT_COMPRESSED 0

/*Enable subpixel rendering*/
#define LV_USE_FONT_SUBPX 0
#if LV_USE_FONT_SUBPX
    /*Set the pixel order of the display. Physical order of RGB channels. Doesn't matter with "normal" fonts.*/
    #define LV_FONT_SUBPX_BGR 0  /*0: RGB; 1:BGR order*/
#endif

/*=================
 *  TEXT SETTINGS
 *=================*/

/**
 * Select a character encoding for strings.
 * Your IDE or editor should have the same character encoding
 * - LV_TXT_ENC_UTF8
 * - LV_TXT_ENC_ASCII
 */
#define LV_TXT_ENC LV_TXT_ENC_UTF8

/*Can break (wrap) texts on these chars*/
#define LV_TXT_BREAK_CHARS " ,.;:-_"

/*If a word is at least this long, will break wherever "prettiest"
 *To disable, set to a value <= 0*/
#define LV_TXT_LINE_BREAK_LONG_LEN 0

/*Minimum number of characters in a long word to put on a line before a break.
 *Depends on LV_TXT_LINE_BREAK_LONG_LEN.*/
#define LV_TXT_LINE_BREAK_LONG_PRE_MIN_LEN 3

/*Minimum number of characters in a long word to put on a line after a break.
 *Depends on LV_TXT_LINE_BREAK_LONG_LEN.*/
#define LV_TXT_LINE_BREAK_LONG_POST_MIN_LEN 3

/*The control character to use for signalling text recoloring.*/
#define LV_TXT_COLOR_CMD "#"

/*Support bidirectional texts. Allows mixing Left-to-Right and Right-to-Left texts.
 *The direction will be processed according to the Unicode Bidirectional Algorithm:
 *https://www.w3.org/International/articles/inline-bidi-markup/uba-basics*/
#define LV_USE_BIDI 0
#if LV_USE_BIDI
    /*Set the default direction. Supported values:
    *`LV_BASE_DIR_LTR` Left-to-Right
    *`LV_BASE_DIR_RTL` Right-to-Left
    *`LV_BASE_DIR_AUTO` detect texts base direction*/
    #define LV_BIDI_BASE_DIR_DEF LV_BASE_DIR_AUTO
#endif

/*Enable Arabic/Persian processing
 *In these languages characters should be replaced with an other form based on their position in the text*/
#define LV_USE_ARABIC_PERSIAN_CHARS 0

/*==================
 *  WIDGET USAGE
 *================*/

/*Documentation of the widgets: https://docs.lvgl.io/latest/en/html/widgets/index.html*/

#define LV_USE_ARC        1

#define LV_USE_ANIMIMG    1

#define LV_USE_BAR        1

#define LV_USE_BTN        1

#define LV_USE_BTNMATRIX  1

#define LV_USE_CANVAS     1

#define LV_USE_CHECKBOX   1

#define LV_USE_DROPDOWN   1   /*Requires: lv_label*/

#define LV_USE_IMG        1   /*Requires: lv_label*/

#define LV_USE_LABEL      1
#if LV_USE_LABEL
    #define LV_LABEL_TEXT_SELECTION 1 /*Enable selecting text of the label*/
    #define LV_LABEL_LONG_TXT_HINT 1  /*Store some extra info in labels to speed up drawing of very long texts*/
#endif

#define LV_USE_LINE       1

#define LV_USE_ROLLER     1   /*Requires: lv_label*/
#if LV_USE_ROLLER
    #define LV_ROLLER_INF_PAGES 7 /*Number of extra "pages" when the roller is infinite*/
#endif

#define LV_USE_SLIDER     1   /*Requires: lv_bar*/

#define LV_USE_SWITCH     1

#define LV_USE_TEXTAREA   1   /*Requires: lv_label*/
#if LV_USE_TEXTAREA != 0
    #define LV_TEXTAREA_DEF_PWD_SHOW_TIME 1500    /*ms*/
#endif

#define LV_USE_TABLE      1

/*==================
 * EXTRA COMPONENTS
 *==================*/

/*-----------
 * Widgets
 *----------*/
#define LV_USE_CALENDAR   1
#if LV_USE_CALENDAR
    #define LV_CALENDAR_WEEK_STARTS_MONDAY 0
    #if LV_CALENDAR_WEEK_STARTS_MONDAY
        #define LV_CALENDAR_DEFAULT_DAY_NAMES {"Mo", "Tu", "We", "Th", "Fr", "Sa", "Su"}
    #else
        #define LV_CALENDAR_DEFAULT_DAY_NAMES {"Su", "Mo", "Tu", "We", "Th", "Fr", "Sa"}
    #endif

    #define LV_CALENDAR_DEFAULT_MONTH_NAMES {"January", "February", "March",  "April", "May",  "June", "July", "August", "September", "October", "November", "December"}
    #define LV_USE_CALENDAR_HEADER_ARROW 1
    #define LV_USE_CALENDAR_HEADER_DROPDOWN 1
#endif  /*LV_USE_CALENDAR*/

#define LV_USE_CHART      1

#define LV_USE_COLORWHEEL 1

#define LV_USE_IMGBTN     1

#define LV_USE_KEYBOARD   1

#define LV_USE_OWN_KEYBOARD   1

#define LV_USE_LED        1

#define LV_USE_LIST       1

#define LV_USE_MENU       1

#define LV_USE_METER      1

#define LV_USE_MSGBOX     1

#define LV_USE_SPINBOX    1

#define LV_USE_SPINNER    1

#define LV_USE_TABVIEW    1

#define LV_USE_TILEVIEW   1

#define LV_USE_WIN        1

#define LV_USE_SPAN       1
#if LV_USE_SPAN
    /*A line text can contain maximum num of span descriptor */
    #define LV_SPAN_SNIPPET_STACK_SIZE 64
#endif

/*-----------
 * Themes
 *----------*/

/*A simple, impressive and very complete theme*/
#define LV_USE_THEME_DEFAULT 1
#if LV_USE_THEME_DEFAULT

    /*0: Light mode; 1: Dark mode*/
    #define LV_THEME_DEFAULT_DARK 0

    /*1: Enable grow on press*/
    #define LV_THEME_DEFAULT_GROW 1

    /*Default transition time in [ms]*/
    #define LV_THEME_DEFAULT_TRANSITION_TIME 80
#endif /*LV_USE_THEME_DEFAULT*/

/*A very simple theme that is a good starting point for a custom theme*/
#define LV_USE_THEME_BASIC 1

/*A theme designed for monochrome displays*/
#define LV_USE_THEME_MONO 1

/*-----------
 * Layouts
 *----------*/

/*A layout similar to Flexbox in CSS.*/
#define LV_USE_FLEX 1

/*A layout similar to Grid in CSS.*/
#define LV_USE_GRID 1

/*---------------------
 * 3rd party libraries
 *--------------------*/

/*File system interfaces for common APIs
 *To enable set a driver letter for that API*/
#define LV_USE_FS_STDIO '\0'        /*Uses fopen, fread, etc*/
//#define LV_FS_STDIO_PATH "/home/john/"    /*Set the working directory. If commented it will be "./" */

#define LV_USE_FS_POSIX '\0'        /*Uses open, read, etc*/
//#define LV_FS_POSIX_PATH "/home/john/"    /*Set the working directory. If commented it will be "./" */

#define LV_USE_FS_WIN32 '\0'        /*Uses CreateFile, ReadFile, etc*/
//#define LV_FS_WIN32_PATH "C:\\Users\\john\\"    /*Set the working directory. If commented it will be ".\\" */

#define LV_USE_FS_FATFS '\0'        /*Uses f_open, f_read, etc*/

/*PNG decoder library*/
#define LV_USE_PNG 0

/*BMP decoder library*/
#define LV_USE_BMP 0

/* JPG + split JPG decoder library.
 * Split JPG is a custom format optimized for embedded systems. */
#define LV_USE_SJPG 0

/*GIF decoder library*/
#define LV_USE_GIF 0

/*QR code library*/
#define LV_USE_QRCODE 0

/*FreeType library*/
#define LV_USE_FREETYPE 0
#if LV_USE_FREETYPE
    /*Memory used by FreeType to cache characters [bytes] (-1: no caching)*/
    #define LV_FREETYPE_CACHE_SIZE (16 * 1024)
    #if LV_FREETYPE_CACHE_SIZE >= 0
        /* 1: bitmap cache use the sbit cache, 0:bitmap cache use the image cache. */
        /* sbit cache:it is much more memory efficient for small bitmaps(font size < 256) */
        /* if font size >= 256, must be configured as image cache */
        #define LV_FREETYPE_SBIT_CACHE 0
        /* Maximum number of opened FT_Face/FT_Size objects managed by this cache instance. */
        /* (0:use system defaults) */
        #define LV_FREETYPE_CACHE_FT_FACES 0
        #define LV_FREETYPE_CACHE_FT_SIZES 0
    #endif
#endif

/*Rlottie library*/
#define LV_USE_RLOTTIE 0

/*FFmpeg library for image decoding and playing videos
 *Supports all major image formats so do not enable other image decoder with it*/
#define LV_USE_FFMPEG  0
#if LV_USE_FFMPEG
    /*Dump input information to stderr*/
    #define LV_FFMPEG_AV_DUMP_FORMAT 0
#endif

/*-----------
 * Others
 *----------*/

/*1: Enable API to take snapshot for object*/
#define LV_USE_SNAPSHOT 1


/*==================
* EXAMPLES
*==================*/

/*Enable the examples to be built with the library*/
#define LV_BUILD_EXAMPLES 0

Your issue is probably just that there are not enough colors between the start and end of your gradient in 16-bit mode.

This LVGL blog posts demonstrates the effect with images and the use of dithering as a solution.
https://blog.lvgl.io/2017-07-03/dithering

It looks like gradient dithering was added in this LVGL pull request and merged into the master branch:
https://github.com/lvgl/lvgl/pull/2872
and can be enabled with LV_DITHER_GRADIENT in your lv_conf.h