I think @ceejayoz has the correct answer.
Your question is really similar to the one answered here: https://stackoverflow.com/a/50578009/6270743
And the answer from Facebook support center for developpers is, I quote :

For dev mode apps, 'App users can only access the data of users who have a role on the app.' https://developers.facebook.com/docs/apps/security Please submit your app for app review and publish your app to access the user info.