My large cube used as a room doesn’t work in @vonpanda’s scene. It bakes the lightmap completely dark, just like it does with me.
I think it’s your model and your normals, then.
In the repo I made the walls have a thickness and checked the normals are pointing inside - but if you just take a cube and put things inside of it, you will have issues:
In Blender, you can see the normals of your model in the “Mesh Edit Mode Overlays”, in the top right of your 3D canvas (only shown while in edit mode)
And you can see the normals, represented by a blue line.
If you just use the default cube, the normals are pointing towards the “outside”.
In comparison, here is the hollow cube I made
If I replace the hollow cube with a default cube, I get this:
If I invert the normals of the default cube, I get the scene to work, but there are artifacts of light bleed through the corners
If I now remove one face of the cube with inverted normals …
It renders properly at first, until I generate the UV2 for it:
Then, after baking the lightmap:
It’s all messed up…
There might, indeed, be a bug in there: I don’t think this is expected behavior.
Tho I am curious, could you show your model’s normals in Blender?
If the example I made works for you, I think you can circumvent your issues by using a totally closed box with appropriate normals. As to how and why the lightmap behaves that way, this is far beyond my knowledge.