If the MovePrimary restarts because of a stepdown (so the phase is not unset) then in the next run it checks again if the target node is draining. If during the stepdown the target started to drain, the check fails, but since the mustAlwaysMakeProgress is very weak here, it retries infinitely.
Meanwile the removeShard cannot finish as it's waiting for the coordinators to finish, so never will let the draining go away.
This causes a deadlock.