I also have this bug in my game. My game generates unions at run time, and this bug happens pretty much every time I generate the parts of my game - which consists of several unions. The way I’m having to combat it is put textures on the sides of the bricks, so they look like they aren’t invisible. Not ideal.
Here is a unanswered thread from June also discussing the problem more specifically:
in the end i just ended up saving the generated parts, and creating maps out of the already made unions
and hello boxing game friend 