Don't know if this is related in cause, but I experienced something similar with icons.
I noticed in a cockpit view, if you move your 'eye' to a spot at the limits of some obstruction, (say just inside some angle iron or whatever) sometimes you can see "through" the obstruction and see the sky and cons, but you won't see the icons. Move your 'eye' outside the obstruction, and the icons return. I noticed this flying the F4U-1 and setting up my head positions.
Could it be related to your head position inside the vehicle Chalenge?