The XSD documentation for the clustering.stateTransfer.fetchInMemoryState setting only mentions nodes joining the cache, it doesn't mention that it also covers the redistribution of state between existing nodes when a node leaves.

The javadoc is a bit more complete/up-to-date, but it only mentions that if disabled, a key may have less than numOwners owners - it doesn't mention that the key might be lost completely, or that some nodes may not be able to retrieve the value of the key even though there is still a copy of the key in the cluster.