I guess UE4 wants everyone to use real-time lighting, effectively declaring that UE4 is for PC/consoles only and mobile/VR platform is a by-product.
Anyhow, I just thought of a hack I’ll try after I get off work. I am planning on placing a small point light with intensity set to 1 inside the room and re-build. That should force Lightmass to calculate insides of the room and hopefully it will be lit more or less properly (dark).
Also thinking about Lightmass Portals, but there isn’t whole a lot of info about that besides: What's New | Unreal Engine 5.3 Documentation