Stripe in display - lv_obj_set_style_bg_color

I built the spi_lcd_touch_example from the esp-idf/exaamples/peripherals directory (under Vscode/ESP-IDF).
I wanted to start programming graphics based on this example to program a CYD2USB Device. It has got a 320x240 pixel display on it (ILI9341 controller with XP2046 type touch). The compiled and flashed example runs fine.

But when I add some code to change the background color, the picture becomes odd:

I added only one line before the invoking of the demo function:

 ESP_LOGI(TAG, "Display LVGL Meter Widget");
    /*Change the active screen's background color*/
    lv_obj_set_style_bg_color(lv_scr_act(), lv_color_hex(0x101010), LV_PART_MAIN);

     while (1) {
        // raise the task priority of LVGL and/or reduce the handler period can improve the performance
        // The task running lv_timer_handler should have lower priority than that running `lv_tick_inc`

How come?