We have a secondary node that is syncing to another secondary that is marked as hidden. My understanding was that hidden would stop the secondary from being used as a sync source.

We have a smallish box created purely for backups. During the backup process the server cannot keep up with replication and so falls behind (but recovers after the backup is finished). As we know that the server gets behind we don't want any of the other servers using it as a replication source.

Further, even if it is picked... once it gets laggy the other server should have the sense to change to a different replication source.