@BillFlowerpot I see what you're describing, but I've never had anything like that in my own game. The only thing I can think of is that while you're building, the game initilally thinks that something you're placing needs support and fills in some tiles before realizing that they're unnecessary. But that's just a wild guess. I could try to recreate your build and the strangeness you're seeing in my own game. I suspect, though, that you'd have better luck asking the build specialists on the Sims forums. There are still quite a few around, and I'd think that someone would know what to make of this issue.
https://forums.thesims.com/en_US/categories/builders
If you can't find a topic that fits your issue (new members can't create their own), I could start one for you. Or, if you prefer, you could post in the new members help thread, although the responders there are more technically oriented. But if it's a graphics card problem, they'd be the right ones to help you.
https://forums.thesims.com/en_US/discussion/893122/thread-for-new-members-to-post-their-sims-3-game-issues/p69
