Upon further investigation, most identity providers are deliberately blocking use of In-App browsers for Authentication because the default in app browser for iOS and Android is not secure.
Read more here: https://auth0.com/blog/google-blocks-oauth-requests-from-embedded-browsers/
As such, this plugin is not appropriate for OAuth interactive login purposes.
The proper iOS way for authentication is to open a ASWebAuthenticationSession (iOS 12+) or SFAuthenticationSession (iOS 11):
https://developer.apple.com/documentation/authenticationservices/aswebauthenticationsession
https://developer.apple.com/documentation/safariservices/sfauthenticationsession
Sadly there is no plugin out there which opens a secure browser so I guess I’m writing one!
Cheers,