The inner and outer walls of this room are all solid
This happens with basic and “optimized” light/shadow settings and lighting quality - nothing wrong with your mesh or placement in world.
You have full answer here A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums or directly here A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums
In very short words, you can check final result only with production lighting quality and this is not all, because you have to tweak shadows quality too. Here a lot of settings and it need some practice, but you should definitely start from studying whole A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums
check to see if there are any spaces once again also be sure to get sphere reflection it should help
also do light build