Xamarin Forms: BaseContentPage and BackButtonBehavior Binding not working (Shell)

Hey guys,

I created my own BaseContentPage and created a BindableProperty for a custom BackButtonBehavior.
My goal is to change the standard backbutton icon and allow a custom BackButtonBehavior with own Commands.

My BaseContentPage looks like that:

    public class BaseContentPage : ContentPage
    {
        public static readonly BindableProperty CustomBackButtonBehaviorProperty =
            BindableProperty.Create(nameof(CustomBackButtonBehavior), typeof(BackButtonBehavior), typeof(BaseContentPage),
                                            null, defaultBindingMode: BindingMode.Default
                                            ,propertyChanged: OnBackButonBehaviorPropertyChanged);


        public BackButtonBehavior CustomBackButtonBehavior
        {
            get
            {
                if (GetValue(CustomBackButtonBehaviorProperty) == null)
                    return CreateDefaultBackButtonBehavior();
                else
                    return (BackButtonBehavior)GetValue(CustomBackButtonBehaviorProperty);
            }
            set { this.SetValue(CustomBackButtonBehaviorProperty, value); }
        }

        public BaseContentPage()
        {
            //Shell.SetBackButtonBehavior(this, CustomBackButtonBehavior);
        }

        private BackButtonBehavior CreateDefaultBackButtonBehavior()
        {
            Application.Current.Resources.TryGetValue("FontAwesomeSolid", out var fontAwesomeSolid);

            var behavior = new BackButtonBehavior()
            {
                Command = new Command(async () =>
                {
                    await Shell.Current.GoToAsync("..");
                }),
                IconOverride = new FontImageSource()
                {
                    FontFamily = ((OnPlatform<string>)fontAwesomeSolid).Platforms.FirstOrDefault(p => p.Platform[0] == Device.RuntimePlatform).Value.ToString(),
                    Glyph = Controls.FontAwesomeSolid.IconConstants.ChessPawn,
                    Size = 22,
                    Color = Color.White
                },
            };

            return behavior;
        }

        static void OnBackButonBehaviorPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {

        }
    }

I use it for a new Xaml Page, which looks like that:

<?xml version="1.0" encoding="utf-8" ?>
<d:BaseContentPage xmlns="http://xamarin.com/schemas/2014/forms"
                   xmlns:d="clr-namespace:IncomingDocumentApp.Controls"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="xxxx.Views.NewItemPage.NewItemPage"
             xmlns:ios="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core" 
             ios:Page.UseSafeArea="true"
             Title="New Item"
             Visual="Material">

than i tried to set the BackButtonBehavior:

   <d:BaseContentPage.CustomBackButtonBehavior>
        <BackButtonBehavior Command="{Binding GoBackCommand}"
                            IsEnabled="True"/>
    </d:BaseContentPage.CustomBackButtonBehavior>

However, it the CustomBackButtonBehaviorProperty is never called.
The Binding is defined in the codebehind of the NewItemPage right after the InitializeComponent() call.

Any help would be appreciated!

snoova