One final insight on what the real cause has to be:
I just noticed after using Unreal’s mesh editing tools that the problem came back. I used Unreal’s mesh editing tools to change the material on the wall to tile originally. This is most likely the cause since the problem didn’t occur until after I did this the first time. Before I re-imported it combined with the ceiling, I fixed the mesh first. It was fine until I went to modify a couple faces on the new mesh and after another lighting rebuild, it had some new weird artifacts. Unreal’s mesh editing tool must have screwed up the lighting UV’s of the wall somehow causing weird light bounces on the ceiling.
I think I’ll be doing all of my mesh editing outside of Unreal from now on