Sounds like RESTer is sending your COOKIES to Twitch and confusing the matter.
Then whatever you are doing must be sending cookies, or doing something weird. Or your library is returned a cached response.
oAuth beats ClientID here. You can get away with no ClientID if you send an oAuth. When you send an oAuth you will get the user data for that oAuth key, it cannot return anything else, unless you are sending something else with your request