I knew trying to work with Oauth was a bad idea…
ASWebAuthenticationSession and at least one swift compiler setting can crash your app.
If Dynamic Actor Isolation(DAI) is on the app will crash when it tries to resolve the callback with your token.
Turn DAI off and it works fine. The default is off.
However in my large project it's on and even turning it off still results in the same crash from another cause I've yet to isolate.
Probably punting on this feature for now.