Changing Root From a Navigation Page

Hi Everyone,

My team and I are using the Login Pattern sample suggested in several answers in these forums

It works great but we need to change this part

    public static Page GetLoginPage (ILoginManager ilm)
    {
                loginManager = ilm;
                return new LoginModalPage(ilm);
    }

    public static Page GetMainPage ()
    {   
            return new MainPage ();
    }

By this

    public static Page GetLoginPage (ILoginManager ilm)
    {
                loginManager = ilm;
                return new NavigationPage(new LoginModalPage(ilm));
    }

    public static Page GetMainPage ()
    {   
        return new NavigationPage(new MainPage ());
    }

Or this at least

    public static Page GetLoginPage (ILoginManager ilm)
    {
                loginManager = ilm;
                return new NavigationPage(new LoginModalPage(ilm));
    }

    public static Page GetMainPage ()
    {   
        return new MainPage ();
    }

Those changes works perfectly in iOS but not in Android. Any suggestions? Is a Bug? Could be fixed?
Our client wants this pattern and no other alternatives for Login because he wants get out of the app from the Login in Android using the hardware back button and he wants several navigation flows in the Login Page, He does not like the Carousel Page approach in the sample.

Any ideas will be appreciated. Thanks for your help.