I believe the retransmitted quantum packet would be necessarily different from the original. So the receiving node notices that the packet has been modified, but because it expects the (secure) hub to modify the original, all is well.
clients can only tell that the message they received from the hub was actually from the hub and was not observed by anyone but the hub. there's no way to tell a message from client A to client B wasn't tampered with or observed before the hub delivered it