I’m struggling to understand what’s happening here. I have a 2x2 ceiling that is showing lighting errors but it’s a very simply mesh with the UV’s all flattened with their own space. Can someone help me out? 
You shouldn’t have those tiles as separate meshes, that ceiling needs to be a single mesh, otherwise the light will not match between them.
We talked about the source of the issue many times in the past but a while ago when the new lightmap UV generator was presented, the developer in charge said he needed to do some research to find a solution.
During the talk a worst-case workaround solution was mentioned; by having a “bake HQ lightmap” feature that would bake the map slower but would produce lightmaps unaffected by the current implementation illness.
I think we need a solution not only because overall the levels will look better but also because in some situation its not convenient or even impossible to combine meshes. Landscape tiles are very problematic right now.
Try to make a night static landscape map with buildings emitting lights. Some funky stuff happen especially when you emit light from emissive materials.
I haven’t seen anything on trello about this but are there still plans to solve it or is it too complicated/problematic to solve right now?
I secretly hope to see improvements about it on every new build ^^
I think we’ll probably get a fully dynamic lighting solution before we get a more improved lightmass.