Administrative boundaries that go along coastlines are discarded by the generator which makes sense.

However its buggy, as some parts of such boundaries are there still.

Actually the algo which should discard those boundaries "tears" them in small pieces instead. This becomes apparent after disabling the short lines filter (it was an experiment in #5655 ):

So at the moment the short lines filter removes most of the "pieces" the buggy boundaries filter produces so overall filtering appears to work though longer line pieces are left. So its not super important, but good to fix someday.

Probably the boundaries filter (in WaterBoundaryChecker::ProcessBoundary()) fails because it compares an already-simplified coastline with a not-yet-simplified boundary line.