Thanks for this test project. Both of the maps have good Surface Cache coverage.
You had Hardware Ray Tracing for the project disabled, but it’s definitely a requirement for clean architecture like this. With Hardware Ray Tracing, the dark ceiling is fixed:
You can see why Software Ray Tracing is so dark if you look at the Lumen Reflection View viewmode, Software Ray Tracing just doesn’t have enough precision to represent the small window openings.
Disabling ‘Affect Distance Field Lighting’ on the window frames opens them up:
Now Software Ray Tracing is giving similar brightness results to Hardware Ray Tracing, but still lower quality.