For example, if the command was "joined" here:

https://github.com/mongodb/mongo/blob/r4.1.5/src/mongo/db/s/shardsvr_shard_collection.cpp#L604

Need to also audit what happens when the command is retried, but fell into the else branch instead of joining like in the above link

the configsvr/shard/mongos versions of shard collection should probably assert that uuid field always exists