Did you add the fireplace yourself? I can't find a house in BP with a fireplace and 1 bathroom and loft. What floor is the invisible hot tub on? If it's on the ground floor, you could try placing some basement, then going down to the basement level and seeing if there is a leftover piece of hot tub there.
EDIT: Could it be the Sugarcube, Loft, 2ba house? There is a fireplace and bookcase there. I didn't see any problems with the house in my game, but you could try deleting the floor tiles where the sims seem to be getting into the 'hot tub'. If you can't delete them then the game must think that there is something there. If that doesn't work, you should try clearing caches and restarting the game to see if that fixes the problem. There is a hot tub outside at that house, near the pool, but not inside.
