Sometimes some gray block is white when grayscale test

[Peter_Wu]

Description

I create 16 level grayscale test. sometimes display blank block .

What MCU/Processor/Board and compiler are you using?

esp32
esp-idf 4.2
lvgl 7.11

What do you want to achieve?

16 level grayscale test well.

What have you tried so far?

Code to reproduce

    lv_style_init(&style);

    lv_color_t color;

    lv_obj_t *obj[16];

    lv_style_set_radius(&style, LV_STATE_DEFAULT, 0); 

    // 16 level of grayscale
    for (int i = 0; i < 16; i++) {

        color = convert_grayscale_to_lv_color(i);   // this is my api, it return LV_COLOR_MAKE(0xii, 0xii, 0xii);   

        obj[i] = lv_obj_create(lv_scr_act(),(i == 0) ? NULL : obj[i-1]);
        
        lv_obj_add_style(obj[i], LV_OBJ_PART_MAIN, &style);

        if (i < 8) {
            lv_obj_set_pos(obj[i], (LV_HOR_RES_MAX / 8) * i, 0);
        } else {
            lv_obj_set_pos(obj[i], (LV_HOR_RES_MAX / 8) * (16 - i - 1),
                           (LV_VER_RES_MAX / 2));
        }

        lv_obj_set_size(obj[i], LV_HOR_RES_MAX / 8, (LV_VER_RES_MAX / 2));

        lv_obj_set_style_local_bg_color(obj[i], LV_OBJ_PART_MAIN,
                                        LV_STATE_DEFAULT, color);
        lv_obj_set_style_local_border_opa(obj[i], LV_OBJ_PART_MAIN,
                                          LV_STATE_DEFAULT, LV_OPA_TRANSP);
    }

Screenshot and/or video

If possible, add screenshots and/or videos about the current state.
correct (70%)

fail (30%) : sometimes some gray block is white

Thanks