Off the top of my head, did you try having only one lightmass volume and extending it over the second room just to check it’s definitely not that? (Edit: actually sorry I think you’re saying that’s already the case?).
I’m not really versed in static lighting (mostly use dynamic) but it looks like the first image uses some kind of skylight or atmospheric fog? Could you perhaps post the list of objects in your scene (top right) just so we can see what’s already there?
Edit 2: You could also perhaps try applying some of the materials from the first room to the second to see if they bounce the light at all. Sorry for the vague suggestions I’m sure it’s probably just a setting somewhere.