Comments

The problem I found is with the wall itself as well as the walls around it. Without going into too much detail, It's not typically a good idea to mix faces like null or sky faces with normal textured faces on the same brush. That being said, simply removing the null texture wasn't enough, afterwards I deleted and remade the wall from scratch and it worked fine. That's it, just delete and remake, and don't use the NULL texture for the back side.