Currently update command only sets _updateOpObj when the `updates` DocumentSequence is present in OpMsg. However this is often not the case, especially due to SERVER-77648 which means that all update commands sent from mongos to mongod are serialized in a way that does not have any DocumentSequence in OpMsg (this can be verified by doing a simple update through mongos in a sharded cluster).
 
Without correctly setting _updateOpObj, the mirrored read requests sent to secondaries will not have filter, hint or collation, and this means that the documents we put into secondary's cache will not be the same as user read on primary. This might have negative impact on the effectiveness of cache warmup.