The following situation is a problem in normal and in transparent server redundancy setups:

It is stronly recommended by OPC UA that a client is reusing a Subscription after a short network interuption by activating the existing Session with the new SecureChannel.

If a client called CreateMonitoredItems during the network interuption and the call succeeded in the server but did not return to the client, the client does not know if the call succeded. He may receive data changes for these monitored items but he is not able to remove them since he does not know the server handle for the monitored item. There is also no way for the client to detect if the create succeeded.

To delete and recreate the subscription is also no option since there may be several monitored items operating normally that should not be interupted.