How to update bulk write when using custom _id?

Question

I have previously not specified an _id when inserting to MongoDB thereby getting an unique MongoDB ObjectId for each document. However, now I am using shortuuid.uuid to genereate unique ids instead, which workes fine for inserting but not for updating. Everytime I have to update I have to comment out the part where I specify _id. It updates and workes fine but it is really annoying having to do that since I need to insert and update quite often.

Bulk write looks like this:

  # Update data
  if data_to_be_updated:

    # We don't want to update the unique id, so we remove it from the item
    item.pop('_id', None)

    collection.bulk_write([
      UpdateOne(
        filter = {'source_id': item['source_id']},
        update = {'$set': item}
      ) for item in data_to_be_updated
    ]) 

and the part that I have specify to use shortuuid.uuid for the _id:

  # Prepare unique mongodb _id (to update, comment this out)
  event_dict['_id'] = shortuuid.uuid()

I know these are just parts of the entire code but maybe it gives an idea to what could be wrong.