It turns out that _local_membership_update doesn't run when you join a new, remote room. It only runs if you're joining a room that your server already knows about. This would explain #4703 and #5295 and why the transfer would work in testing and some rooms, but not others. This would especially hit single-user homeservers.
The check has been moved to right after the room has been joined, and works much more reliably. (Though it may still be a bit awkward of a place).
Andrew Morgan