Hello -
The navigation system taken from a sample (I cannot post a link: developer.xamarin.com > /guides/xamarin-forms/getting-started/hello-xamarin-forms-multiscreen/quickstart/) works on one of my phones, while it doesn't fully work on another.
The part that does not function is the OS-provided back-button: it does nothing, even though the button itself provides the "tap" feedback. Everything is exactly the same, as far as app code and configuration, between the two phones. Pressing the back-button on the "working" phone acts as if I pressed the back-arrow located within Xamarin-provided navigation bar on the top of the screen.
The "non-working" phone is LG G6, running Android 7.0. I'm not sure if its software-navigation bar is a part of OS or LG's launcher. The phone's screen has round corners, with the concept of classic full-screen possibly changed somewhat. I don't know how to explain it briefly; if anyone is interested, I can try to provide some more details and screenshots. The point is, there could be some new event related to this that Xamarin's navigation isn't paying attention to.