Are your joists sitting above the ceiling slab?
If you have two surfaces in exactly the same plane you will have this issue - i.e. the bottom of your joist are in line with the bottom of the slab.
I have seen this in the OpenGL window even when the surfaces are close but not touching, but have not noticed it in BIMx.
Is it an option to not include the joist layer in the BIMx if they are not to be seen?
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10