Seems like this isn't getting much activity but just in case someone runs across this bug.
create_user is not setting the key which defaults to setting an id. I extended the user model and overrode the create_user class method to set an entity key to the auth_id. You'll need to override add_auth_id to update the key to the new auth_id.
user_values['auth_ids'] = [auth_id]
user_key = model.Key(cls, auth_id)
user = cls(key=user_key, **user_values)