A shard can be added with an arbitrary name. E.g. as tested in jstests/sharding/names.js, we can even add replicaSetA using as shard name replicaSetB, and the inverse, replicaSetB using as shard name replicaSetA.

As a clean up step after removeShard, we call ReplicaSetMonitor::remove(name). But the name variable is not the replica set name, but the shard name. This can lead to incorrectly removing the RSM monitor for a shard which has not been removed. This was hidden by the fact that the balancer forces a ShardRegistry refresh in every round (SERVER-91974), and before SERVER-91121 because the ShardRegistry was periodically forcing a refresh.