The logic in the shard registry to incorporate new nodes into the connection strings stored in the shard registry triggers a forced refresh when we receive a new connection string. However, the updateReplSetHosts function in the shard registry is actually called on each possible and confirmed set. This triggers based on the hasMembershipChange() function, but this function is broader than just nodes being added and removed, and is also triggered for state changes.

The implication of this is that any replica set state change will cause a forced refresh of the shard registry on all nodes once they hear of it.