mongocxx::pool mongos failover

We have shard cluster with 3 mongos instances for failover purposes. All mongos in future, will be distributed on different hosts (now they running on same host with different ports for test purposes).
The failover needed for backup network channels between client app and mongos. For this purpose, we provide all mongos addreses in mongocxx::uri and then pass it to mongocxx::pool.
Now, we see so mongocxx::pool does not failover to another mongos if we shutdown first (for example).
But mongocxx::client does it well (failover to different mongos).

Does this behaviour is correct or maybe we need to additional setup mongocxx::pool environment for the mongos failover?