Dictionary index binding does not detect changes #20313

Describe the bug

When I bind an entry of an observable dictionary using "{Binding Dictionary[Key]}", it does not detect changes.

To Reproduce

• Create a new Avalonia MVVM app.
• Add a view model property with an AvaloniaDictionary (or other observable dictionary).

public partial class MainWindowViewModel : ViewModelBase
{
    ...
    public AvaloniaDictionary<string, string> Dictionary { get; } = new();
    ...
}

• Bind a dictionary entry in the view.

<Window ...>
    ...
    <TextBlock Text="{Binding Dictionary[MyKey]}" />
    ...
</Window>

• Now programmatically change the value of Dictionary["MyKey"].

Result: The UI does not detect the change. The TextBlock.Text is not updated.

Expected behavior

The AvaloniaDictionary implements INotifyPropertyChanged and INotifyCollectionChanged, therefore I expect the UI to detect changes and update the UI text.

Avalonia version

11.3.10

OS

Windows

Additional context

It works when the observable dictionary raises a PropertyChanged event with new PropertyChangedEventArgs("Item"). It does not work with new PropertyChangedEventArgs("Item[]").

Note that binding an ObservableCollection with {Binding List[0]}" is working fine. It is only a problem with dictionaries.