Understanding how the MEAN stack mongo driver operates can be tricky, as it often automatically modifies objects after a successful insert operation by adding a new ObjectId or _v field. This generation of ObjectId is typically handled by the driver itself rather than the MongoDB server.
In some cases, simply returning the same object after a post call may provide you with the generated ObjectId. It might be worth printing the object to the console after insertion to confirm whether the driver is handling this process for you.
If the driver isn't automatically updating the object as expected, then unfortunately, you may need to come up with an alternative solution.