To illustrate the problem I created a custom control PingPongView with the following behavior:

• if the Ping button is pressed then a star * is appended to its Message.
• if the Pong button is press then the last character in Message is removed.

public partial class PingPongViewModel : ObservableObject
{
    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(PongCommand))]
    string _message = string.Empty;

    bool IsMessageNotEmpty => Message != string.Empty;

    [RelayCommand]
    void Ping() => Message += "*";

    [RelayCommand(CanExecute = nameof(IsMessageNotEmpty))]
    void Pong() => Message = Message.Substring(0, Message.Length - 1);
}

public partial class PingPongView : ContentView
{
    public static readonly BindableProperty MessageProperty =
        BindableProperty.Create(
            propertyName: nameof(Message),
            returnType: typeof(string),
            declaringType: typeof(PingPongView),
            defaultBindingMode: BindingMode.TwoWay,
            propertyChanged: OnMessagePropertyChanged);


    public string Message
    {
        get => (string)GetValue(MessageProperty);
        set => SetValue(MessageProperty, value);
    }

    static void OnMessagePropertyChanged(BindableObject bindable, object oldValue, object newValue)
    {
        var view = (PingPongView)bindable;
        view._model.Message = (string)newValue;
    }

    PingPongViewModel _model;
    public PingPongView()
    {
        InitializeComponent();
        _model = new PingPongViewModel();
        BindingContext = _model;
    }
}

<ContentView ... others are trimmed ... 
             x:DataType="loc:PingPongViewModel">
    <HorizontalStackLayout >
        <Button Text="Ping" Command="{Binding PingCommand}" />
        <Button Text="Pong" Command="{Binding PongCommand}" />
    </HorizontalStackLayout>
</ContentView>

I used PingPongView in MainPage as follows:

<Entry x:Name="Entry" Text="0" />
<loc:PingPongView Message="{Binding Text,Source={Reference Entry}}" />

<Entry Text="{Binding Message,Source={Reference PingPongView}}" />
<loc:PingPongView x:Name="PingPongView" />

Unfortunately in both cases, the Text of Entry controls don’t reflect changes in PingPongView. What am I wrong here?