Seemingly random 403 Error (even for users added to dev dashboard)

[heyitsmyry]

Heya!

I'm developing a web API. I ask β testers to give me their email when they try to run my code so I can add them to the dashboard. If they run my code before I've added the email, I still get an accessToken and refreshToken 200 (and get the tokens) but the requests to user-read-private (to get basic profile info and therefore confirm their email was correct, since a lot of people get confused) SOMETIMES fail regardless of whether I had or hadn't added them to the developer dashboard yet.

I have a suspicion this could be related to people logging in with facebook/gmail and that those users MUST be added before whereas other users can be added after?

The unpredictable response is causing a lot of confusion and I'm going in loops trying to fix my code around it. Would be very grateful for any help!