Just for testing purposes, I tried spawning the walls in as components using transform (instead of using an instanced static mesh). The slits were no longer present. This means you are right, this does not appear to be an issue with the rotation.
I’m not sure the issue would be with the lighting or any external source, since swapping the instanced static mesh for individual components seems to fix the issue. Perhaps it is a bug with instanced static meshes? I did find another thread that suggests there is some scaling bug with instanced static meshes.