When a new cluster node is joining the cluster, its MUC room state is merged with that of the same rooms in the cluster.
There can be a scenario where the joining node has an occupant that uses a nickname, for which the nickname is already in use by a different occupant on another node. The existing implementation does not handle this (and will simply overwrite internal state of Openfire). Instead, proper handling (that likely involves communication to a client) needs to be implemented.
Note that a related, but distinct scenario is where each cluster node has a different occupant, but that those represent two different resources of the same end-user (Multi-Session nicks).