$merge relies on the old batch write executor's use of the CollectionRoutingInfoTargeter to detect collection drops between rounds, as the CRIT stores a _targetEpoch for namespaces and checks that it's the same on routing context initialization, throwing a StaleEpoch error if it's not (here, caught here) which is then converted into QueryPlanKilled in the merge stage. We need to do a similar check in the UWE.