Build mode: An empty basement level is visually buggy

@Zipanu  I've moved your post to the Mac section for now, since it's not clear whether this is a game bug or a Mac-specific issue.  Thanks for your specs, by the way; the info is always helpful.

Could you post the graphics settings you're using?  You can just take a screenshot of the page where you select settings.  I want to see whether I can reproduce this in Windows, and maybe macOS if I can get my game running at some point.