UpdateOne With Upsert True and InsertOne Behavior During BulkWrite

Hello, I have some questions regarding UpdateOne and InsertOne behavior when performed in the same unordered BulkWrite operation or separately in 2 BulkWrites but executed at the exact same time.

For example, if I perform the 2 operations below in a single unordered BulkWrite:

db.collection.insertOne(
    {
        "_id": "A",
        "status": "in_progress",
        "...": "..."
    }
)

db.collection.updateOne(
    { "_id": "A" },
    { 
        "_id": "A",
        "status": "success",
        "...": "..."
    },
    { upsert: true }
)

I would assume that, after execution, the resulting document would always have a status of success. However, after load testing, I still find some documents with an in_progress status.

My questions are:

1. What is causing this behavior? Looking at the operation, I would assume that either the insert is performed first and would then be overridden by the update or the insert is performed second which would throw a duplicate key exception due to the unique index _id.

2. Is there any way to guarantee that the data in the UpdateOne would always be the one persisted in the resulting document?

I would appreciate any help. Thank you.