I don’t know why it happens, but I can enter the base and see the thralls & pets standing in the foundations, as if they are on the ground and the foundation was poured around them. Sometimes the thrall following me will get stuck in the foundation.
Also, in the club, the dancers don’t always remain on the table tops. They have fallen through to the floor, and are dancing in the table, not on it. (Yes, we have a nightclub, why don’t you? The dancers are on tables, because why not?)
If I re-place them, they will stay in the right place for a while, but they always fall through again.