I just made a quick test on your test template on the UE4 4.12.5, and got this:
A view from the inside, the bright floor problem is clearly visible:
After i made a production building with no change or adjustments:
I can’t get looking into it further at the moment as i’m a bit strained to my time limits, sorry(!), but the unwrapped lightmap channel UVs doesn’t seem to be looking good (pixel/space between the uv’d sides):