Same problem. It doesn't seem like the authentication handler is being called. I get an error that I'm trying to present a view controller when there's already one being presented, which I assume is the login view controller, but I don't seem to have any way to get the login view controller on my own before the error happens.

It seems like Apple used to trust us to show the login view controller in the course of authentication, but now it just forces the login controller on us on its own. This is causing me crashes, because Apple tries to make my root controller present the login controller, even though my root controller is already presenting a different controller. So it's forcing a view controller on me, without warning, and then blaming me for not being able to present it. Not cool!