client = MongoClient(uuidRepresentation='standard')
collection = client.test.test
# normal insertion gives UUID representation
result = collection.insert_one({'_id': uuid.uuid4()})
result.inserted_id
>>> UUID('1ec97f53-b1ae-471b-9e0c-157ca9b02953')
doc = {'_id': uuid.uuid4()}
result = collection.replace_one(doc, doc, upsert=True)
result.upserted_id
>>> UUID('c03ba52b-e594-47ca-8f12-249ae8eabf05')
# bulk gives binary representation (this is the bug)
doc = {'_id': uuid.uuid4()}
result = collection.bulk_write([ReplaceOne(doc, doc, upsert=True)])
result.upserted_ids
>>> {0: Binary(b'\xb6\r%\xf2\xadOMo\x919\xc4\x98P\xe5\n\x02', 4)}