just to debug, can you place the same two meshes next to eachother (same config) but not the BP versions, just regular static meshes dropped into the scene. Also same thing with the lights, place the same lights but not BP ones, regular level placement.
Do this test and see if the problem remains, at least to check it’s not a BP problem when there’s lights nested in them.
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”.
