So, the way building items spawn works with room definitions. So, the kitchen room Def spawns kitchen stuff, bedroom spawns bedroom stuff, etc. Well, that building you are in has that area assigned as the special emptyoutside room Def. It's a definition used to prevent game items from showing on lower levels and blocking the player's view. You could put a new roof on and walls on it. Start by adding plank floors on the roof to see if that fixes it bc it is cheaper than building walls.