As a new “feature” of iOS 11, Safari web views no longer share cookies with the main Safari app.
As a result, when you upgrade to iOS 11, you’re logged out of all of your associated forums in the Discourse iOS app. When you attempt to login with Google or Facebook, your authentication credentials aren’t there; you have to sign in to Facebook with a password, re-enter your second factor code, and “remember this browser” in the future.
The intended workaround for this is for iOS apps to use SFAuthenticationSession for logging in to third party services.
The Discourse iOS app should use SFAuthenticationSession to sign in to Discourse forums.