Its back... took out my relatively small mod folder and made sure I disabled CC and Script mods in the game settings just to be 100% sure it wasn't a conflict from any CC/script, relaunched the game and bug persisted.
Like always its a rental lot, and trying to place or alter a roof gets the "Not Expanded to Ops" error message. And the only short term fix is to delete every single staircase in every single unit (which i could only do by unjustly evicting my sim's bestie and now shes got a grudge). Afterwards the bug returned, so I can't even continue building the rest of the houses on the lot.
Do I just scrap the whole thing?