Re: [NOT A BUG] Split levels/Lofts - trouble with Glass Roofs
I don't think this is an error at all. The problem in the split-level house is that, by deleting the wall between rooms, you essentially "explode" the floorless room so it is, instead, an open outdoor space with three free-standing walls around it. If, instead, you were to replace the wall with a fence, the floorless room will remain a room and the artifact will not appear. You can tell by clicking on any of the remaining walls of the floorless room: does the whole space get highlighted, or only that one wall?
It's the same story with the double-storey loft space. Deleting a wall will explode the room; that's what's causing the flooring to reappear. (Edit: that's not actually the flooring of your upstairs room: that's the ceiling of the room underneath.) Do NOT delete walls; replace them with fencing instead, or with spandrels.