- Are walls are too thin?
Walls should be no thinner than 10 centimeters (cm) to avoid light leaking.
https://docs.unrealengine.com/5.0/en-US/lumen-technical-details-in-unreal-engine/#softwareraytracing
- Light Leaking in Large Cave-like Areas
In cave-like, or enclosed areas, if light leaking is happening, it is because Lumen limits how far rays are traced to improve performance, but missing an occluder causes light leaking.
Solution:
In the Post Process Setting, raise the Lumen Scene Detail and the Max Trace Distance.
https://docs.unrealengine.com/5.0/en-US/lumen-technical-details-in-unreal-engine/#mismatchbetweenshadowmapsandray-tracedshadows