Currently there is an optimization in the chunk migration code where if the source sees that an oplog entry for a session has a different namespace than the one being migrated, it skips inspecting the rest of the oplog chain, with the assumption that all entries in the chain will be for a single namespace.

Unfortunately that assumption no longer holds up for bulkWrite since a single retryable write can now touch multiple namespaces. And so if for example we have an oplog chain (from newest to oldest) generated from a single retryable bulkWrite with:

1. write to coll1
2. write to coll2

And we are migrating coll2, we would see that the first entry is on a different ns and skip the rest of the chain.