Draw line on canvas not shown if the line is vertical (x1 = x2)

With Simulator in Windows on v8.3.

The code to draw a vertical line in the canvas:

#define CANVAS_WIDTH  469
#define CANVAS_HEIGHT  415

static lv_color_t cbuf[LV_CANVAS_BUF_SIZE_TRUE_COLOR(CANVAS_WIDTH, CANVAS_HEIGHT)];

lv_obj_t* canvas = lv_canvas_create(lv_scr_act());
lv_canvas_set_buffer(canvas, cbuf, CANVAS_WIDTH, CANVAS_HEIGHT, LV_IMG_CF_TRUE_COLOR);

lv_obj_center(canvas);

lv_draw_line_dsc_t line_dsc;
line_dsc.color = lv_color_hex3(0x999);
line_dsc.width = 3;
line_dsc.opa = LV_OPA_COVER;

lv_point_t pts[2];
pts[0].x = 20;
pts[1].x = 20;           // 20 cannot work, but 20 + 1 works
pts[0].y = 30;
pts[1].y = 500;

lv_canvas_draw_line(canvas, pts, 2, &line_dsc);     // vertical

If pts[0].x = pts[1].x = 20, the line cannot be shown.
If I tried to set the pts[1].x other value to be different from the pts[0].x, assume 20 and 21, the line can shown.

Is there any problem with the vertical line ? Thx.