[BDK] Can't vacuum < 4 floor tiles - tiles still get dirty

I play rotation and have families using both the prebuilt underground boathouse in Sulani and the under the plane house in Strangerville.  Both houses have very tiny entryways on the ground floor before going down to the basement.  I had expanded the boathouse one to be able to vacuum it (decreasing the stairs and widening the wall and expand the room as much as possible to be able to vacuum.  I took a couple of months' break, and now I can't vacuum either.  The boathouse one doesn't even look dusty...