I only notice some thralls disappearing up to their waists and sometimes heads when on ceiling pieces and not pure foundation pieces. I suspect (with no basis of evidence) that this is undermesh related and would be included in the many bugs that will be addressed in the next update/fix. I got mine out by deleting a ceiling and dragging the guilty party out - not idea as that ceiling was supporting a wall, some roof, and a weapon in a display bracket I had forgotten about. Yay… lol