I don't understand why this light is leaking

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