Workaround: handle the RadPropertyChanging and RadPropertyChanged

public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();
        
        this.layoutControlGroupItem1.RadPropertyChanging += LayoutControlGroupItem1_RadPropertyChanging;
        this.layoutControlGroupItem1.RadPropertyChanged += LayoutControlGroupItem1_RadPropertyChanged;
    }

    private void LayoutControlGroupItem1_RadPropertyChanged(object sender, RadPropertyChangedEventArgs e)
    {
        if (e.Property == LayoutControlGroupItem.IsExpandedProperty)
        {
            Console.WriteLine("IsExpanded Changed");
            Console.WriteLine("Old Value: " + e.OldValue);
            Console.WriteLine("New Value: " + e.NewValue);
        }
    }
}