In a network of two brokers, a durable subscription is correctly propagated across to the remote broker. However when the consumer unsubscribes from the durable subscription again, it is only removed on the local broker but not on the remote broker. The remote broker keeps its durable subscription alive.
As a consequence messages sent to the topic destination on the remote broker for which the durable subscriptions existed, are passed on to the local broker, although there is no active subscription on the local broker. The local broker will discard these msgs but unnecessary traffic has already occurred on the network bridge.