Clustering: DataTreeCandidateInputOutput IAE: Unhandled node type 2

Controller: opendaylight/md-sal/sal-distributed-datastore/src/main/java/org/opendaylight/controller/cluster/datastore/persisted/DataTreeCandidateInputOutput.java
contains two codec-like methods: writeDataTreeCandidate() and readDataTreeCandidate().
While write* is aware of type 2 (UNMODIFIED), read* does not and throws this IllegalArgumentException.

This is happening (along 2 other symptoms) in Carbon Netconf clustering all job, so it is not clear which component leads to UNMODIFIED candidate being persisted. The analogous only job does not encounter this symptom, so perhaps Netconf is not the component.

One of three symptoms cause system test failures. First failure is when deconfiguring a netconf device prevents subsequent mount of a device of the same name.