CollectionUUIDMismatch today is populated with the actualCollection namespace almost universally by contacting the dbPrimary shard in the event of a failure.

However, the only place that doesn't do this is the find command on mongos. If it knows that the request targeted the dbPrimary it assumes that the error has been already populated by the dbPrimary response and skips the call to it.

However, if the primary shard returns an error different than CollectionUUIDMismatch then the error isn't populated at all and returns actualCollection=null even if it should be something different.