When on the debug screen, the crosshair will switch to being 3 short colored lines to indicate the direction of each axis. However, if you are in 2nd or 3rd person mode, it will instead appear as a small dot. As you move the viewpoint around, it will change color. What appears to be happening is that the crosshair is still the 3 lines, but is rendering much smaller, so you only see it as a few pixels.