NavigationController missing back button when coming from the third level to the second level

I have a multiple screen application. From the level1 screen you can access the level2 screen then the level3 screen. My problem is when I come back from the level3 screen to the level2 screen. The back button is missing (see image) in the level2 screen, but only if I come from the level3 screen. Can you help me please?

AppDelegate.cs

namespace MultiLevelApp
{
    [Register ("AppDelegate")]
    public partial class AppDelegate : UIApplicationDelegate
    {
        UIWindow window;
        UINavigationController navController;
        DialogViewController level1ViewController;

        public override bool FinishedLaunching (UIApplication app, NSDictionary options)
        {
            window = new UIWindow (UIScreen.MainScreen.Bounds);
            window.MakeKeyAndVisible ();
            navController = new UINavigationController ();

            level1ViewController = new Screens.Level1Screen ();
            navController.PushViewController(level1ViewController, false);
            window.RootViewController = navController;
            window.MakeKeyAndVisible ();

            return true;
        }
    }
}

Level1Screen.cs

namespace MultiLevelApp.Screens
{
    public partial class Level1Screen : DialogViewController
    {
        public Level1Screen () : base (UITableViewStyle.Grouped, null)
        {

        }

        public override void ViewWillAppear (bool animated)
        {
            base.ViewWillAppear (animated);

            StyledStringElement element = new StyledStringElement ("Go to Level2", () => {GoToLevel2 (); });
            Section section = new Section ();
            section.Add (element);
            Root = new RootElement ("Level 1") { section };
        }

        protected void GoToLevel2()
        {
            DialogViewController level2Screen = new Screens.Level2Screen ();
            NavigationController.PushViewController(level2Screen, true);
        }
    }
}

Level2Screen.cs

namespace MultiLevelApp.Screens
{
    public partial class Level2Screen : DialogViewController
    {
        public Level2Screen () : base (UITableViewStyle.Grouped, null)
        {

        }

        public override void ViewWillAppear (bool animated)
        {
            base.ViewWillAppear (animated);

            StyledStringElement element = new StyledStringElement ("Go to Level3", () => {GoToLevel3 (); });
            Section section = new Section ();
            section.Add (element);
            Root = new RootElement ("Level 2") { section };
        }

        protected void GoToLevel3()
        {
            DialogViewController level3Screen = new Screens.Level3Screen ();
            NavigationController.PushViewController(level3Screen, true);
        }
    }
}

Level3Screen.cs

namespace MultiLevelApp.Screens
{
    public partial class Level3Screen : DialogViewController
    {
        public Level3Screen () : base (UITableViewStyle.Grouped, null)
        {

        }

        public override void ViewWillAppear (bool animated)
        {
            base.ViewWillAppear (animated);

            Section section = new Section ();
            section.Add (new StyledStringElement ("this is the last level"));
            Root = new RootElement ("Level 3") { section };
        }

    }
}