Hi mikegarn,
With this area looking indirectly lit the shadowing differences you’re seeing is caused by lighting for each actor being handled by a different thread when it’s computed. It’s harder to see with the with the world grid material, so if you use a base color like white it’s much easier to see the disparity between the pieces.
This can help you from the wiki for lighting troubleshooting and tips with some suggestions to improve this or alternative options to take: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
-Tim