Here is the thing, some buildings are symmetric (if not most) and drawing them is a piece of cake, mirroring becomes your best friend command.
In VA mirroring works fine, but sometimes it produces such artefacts:
They reappear only if you move nested objects around.
This seems to be a bug in VisualARQ. However, I’ve been testing to mirror on some models with walls and I’m unable to reproduce your problem. Probably it only happens in particular cases. Can you please send me a model where you’re seeing this bug to email@example.com?