Since upgrading to pymongo 3.9 we are seeing documents that are failing to insert or update using the following pymongo calls:

coll.update_one(flattened_json_data, {{color:#6a8759}'$set': flattened_json_data}, upsert=True)

coll.find_one_and_update(query, update, upsert=True, projection=projection, return_document=ReturnDocument.AFTER)

However, pymongo seems to be returning successfully even when documents aren't inserted or updated.

We believe this might be due to the fact that we are not using the MongoClient in a fork safe way. We spawn threads and processes that all use the same shared MongoClient connection and sometimes causes the error described here: https://jira.mongodb.org/browse/PYTHON-1992?jql=text%20~%20%22Cannot%20start%20transaction%20on%20session%22

However, is it possible that due to the issues with our application logic's unsafe use of the MongoClient, pymongo could return successfully without having actually successfully completed the insertion/update?

We have since fixed our application logic and have stopped seeing the error described in the linked PBI. We are hoping the issue of failed inserts will be fixed as well.