When drawing lines of equal start and end x, y coordinates the GFX library shall draw a rectangle with thickness applied. If the value of the opposing axis has a start value greater than the end value this results in a line drawn in the opposite direction. Tested with nRF5 SDK 15.0.0 a53641a.
Here is a patch which uses MIN() for drawing the rectangle (line):
--- components/libraries/gfx/nrf_gfx.c 2018-03-22 15:25:07.608481800 +0100 +++ components/libraries/gfx/nrf_gfx.c 2018-03-22 15:25:07.608481800 +0100 @@ -265,8 +265,8 @@ if ((p_line->x_start == p_line->x_end) || (p_line->y_start == p_line->y_end)) { rect_draw(p_instance, - p_line->x_start, - p_line->y_start, + MIN(p_line->x_start, p_line->x_end), + MIN(p_line->y_start, p_line->y_end), abs(p_line->x_end - p_line->x_start) + x_thick, abs(p_line->y_end - p_line->y_start) + y_thick, color);