Hey guys! Sorry about the delayed response. I went back and spent some time messing with this, trying to figure out where and when I’m going wrong. So far I’ve:
-Constructed a new scene out of closed, watertight, non-interpenetrating meshes
-Cranked the lightmap resolution up to 2048
-Build lighting on Production quality
Unfortunately I’m still getting the same problem:
It’s got a roundish falloff that’s much bigger than the texel size–so I don’t think it’s an issue of bleeding across the lightmap UV:

Not to mention this ridiculous issue when I stand near it, haha:
It looks like it’s somehow bouncing around inside the mesh and then bouncing through the backfaces on the other side, even though the wall mesh is now closed and perfectly flush with the floor and walls. Is there anything I can do to prevent this besides making the walls all cartoonishly thick?

