Setting MainPage = NavigationPage during OnStart in Xamarin.Forms displays a blank screen in Android

I have a Xamarin Forms Application where I'm setting the MainPage to NavigationPage during OnStart.
The NavigationPage is static (kept in memory for reuse).
When I press the back button in Android and go back to the app the OnStart method is executed again and the application displays a blank screen.

See the repro here: Xamarin.Forms OnStart Navigation

public partial class App : Application
{
    private static readonly NavigationPage NavigationPage = new NavigationPage(new MainPage());
    public App ()
    {
        InitializeComponent();
    }

    protected override void OnStart()
    {
        MainPage = NavigationPage;
    }
}

If I do the same action in the constructor the application behaves as expected: there's no blank screen and the last page visited is displayed.

See the repro here: Xamarin.Forms Constructor Navigation

public partial class App : Application
{
    private static readonly NavigationPage NavigationPage = new NavigationPage(new MainPage());
    public App ()
    {
        InitializeComponent();

        MainPage = NavigationPage;
    }
}

What is the difference between setting the MainPage in the constructor and in the OnStart method?

You do not do it in onstart with static mainpage intialized outside the init call.

Do it in constructor after the InitializeComponents.

Like so:

public partial class App : Application
{
private static readonly NavigationPage NavigationPage;
public App ()
{
    InitializeComponent(); 
    NavigationPage = new NavigationPage(new MainPage());

    MainPage = NavigationPage;
}
}