Welp, the quick answer is, “I don’t know.”
I just setup a temp project and replicated your results. I tried importing a custom cube I made and got shadow errors on it as well.
If you make your walls one object in an external 3d program, lightmap it correctly and drop it in, any weird shadow thing is going to be in the corners where it would be least noticable.
Sorry I couldn’t be more helpful :^\