How to reproduce: check the attached project and video

Workaround:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        this.LostFocus += RadForm1_LostFocus1;
    }

    private void RadForm1_LostFocus1(object sender, EventArgs e)
    {
        //Workaround
        this.radMenu1.GetType().GetField("shouldShowChildren", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).SetValue(this.radMenu1, false);
    }
}