Some write commands run on sharded collections return different values compared to replica-sets. See the TODOs in jstests/libs/write_commands.js as they are tested via jstests/sharding/mongos_writes_wait_for_write_concern.js – tests written in SERVER-80103.

• Duplicate 'update', mongos returns n:0. In replica-set, returns n:1.
• 'update' on multiple docs, mongos returns n:0, nModified:0. In replica-set, returns n:2, nModified:2.
• 'createIndexes' where index already exists, mongos returns numIndexesAfter:2. In replica-set, returns numIndexesAfter:1.
• 'insert' where document with same _id already exists, mongos returns n:1. In replica-set, returns n:0 and writeErrors:DuplicateKey.
• 'findAndModify' where the update violates unique index or attempts to change immutable field, mongos returns ok:1. In unsharded coll and replica-set, returns IllegalOperation or ImmutableField error.