In general NiFi does its best to prevent changes being made to the flow when one of the cluster nodes is down. For example, if you have a 3 node cluster and only 2 nodes are up, you can't change the flow.
When a request comes in to change the flow, lets say you drag a new processor on the graph, this is sent to one of the nodes which then does a two phase-commit with the rest of the nodes in the cluster.
The error message you got means that all the nodes responded successfully to the first phase, and on the second phase of the commit, one of the nodes encountered an error.
At this point the change was applied to the other nodes, and the node with the error was purposely disconnected from the cluster because it is in an inconsistent state.
If possible, can you see what other errors happened in the log of that node before seeing "host out of cluster..."? because the real problem is there was some other issue on that node that caused it to fail.