Messy build mode error

The other day, I decided to update my game with a new feature. Everything seemed to be working fine until I entered a family and went into build mode. This is the first time I've encountered this problem. I have downloaded a lot of custom content, but I don't think that’s the issue. Can anyone help me with this problem? I love designing my Sims' houses, and this issue is making it difficult for me to enjoy the game.