Invisible walls after editing house.

[Mama Murphy]

I've been trying to add some decorations (paintings) into a custom house mod but I keep running into this problem. Everything is looking good up until I place the paintings into the game. After that I notice the walls around the stairwell are invisible/glitched. I'm not sure what I'm doing wrong as I made it a point not to touch anything else. Also, the walls that are going invisible are not even the walls that I put the paintings on.