From the dev list:
There's an optimization that when we're lucky enough that the paired view replica is the same as this base replica, mutateMV doesn't use the normal view-mutation-sending code (wrapViewBatchResponseHandler) and just writes the mutation locally. In particular, in this case we do NOT write to the pending node (unless I'm missing something). But, sometimes all replicas will be paired with themselves - this can happen for example when number of nodes is equal to RF, or when the base and view table have the same partition keys (but different clustering keys). In this case, it seems the pending node will not be written at all...
This was a regression from
CASSANDRA-13069 and the original behavior should be restored.