Wild guess…the “T” vs the “O”. With the “O” the lighting calculation is bouncing light only off of, effectively, 8 ‘sides’. With the “T” it is bouncing off of a lot more…the connection part, and some of the walls of the other corridor sections. Maybe the Lightmapping is bouncing it around enough that it ‘brightens’ the whole thing? I mean, if you take a flat plane and put a green light on it, then build the lightmap, you get a green light with dark, almost black (if not black) outside the lights radius. If you put a wall on one side, you get distinctively lighter floor area (more light bounces). Add another wall, get more light. Maybe this is the thing going on?
Try putting a full ‘wall’ blocking off one side of the “O”, and a small, half-wall on the other side…then build. See if you see it brighten up like the “T”.
Just a thought…
Paul L. Ming