flask requests are not thread safe when we run it with gunicorn

Question

I am running a flask application and using flask-RESTPlus as an extension to build REST APIs.

I have a user API that retrieves user information from postgres and also reads related information Redis and sends back to the client.

I am running my server with gunicorn like gunicorn application:app --workers 1 --threads 12 -b 0.0.0.0:5000

now if I make multiple parallel requests then it's returning one user information into response of another user request. lets say if I make parallel requests for user A, B and C then its sending information of user A into the response of request B.

If I remove --threads 12 then it works fine.

I have a huge code base so couldn't add it but extracted some useful code so you guys can imagine it

@USER_API.route("/<string:user_id>")
class UserResource(Resource):
  def get(self, user_id):
    # fetch user from DB
    user = User.find(user_id)
    keys = ["address", "office", "xyz"]
    relations = [
            json.loads(fleet) for fleet in list(filter(bool, r.mget(*keys)))
        ]
    # call function to add redis datta into user obj
    user = add_relation_to_user_obj(user, relations)
    return user