e.g. Open the artist credit editor on https://beta.musicbrainz.org/release/a6a265bf-9f81-4055-8224-f7ac0aa6b937/edit , click the remove button for the first row and the last join phrase gets removed.

I assume this is related to the defaults (so that "A, B, C & D" turns into "B, C & D" if you delete the first row and "A, B & C" if you delete the last row), but it should presumably only change them if the join phrases actually follow that pattern.