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

        this.commandBarDropDownButton2.DropDownMenu.DropDownOpening += DropDownMenu_DropDownOpening;
    }

    private void DropDownMenu_DropDownOpening(object sender, CancelEventArgs e)
    {
        if (this.WindowState == FormWindowState.Maximized)
        {
            RadDropDownMenu dMenu = sender as RadDropDownMenu;

            RadPopupOpeningEventArgs eventArgs = (RadPopupOpeningEventArgs)e;
            eventArgs.CustomLocation = new System.Drawing.Point(eventArgs.CustomLocation.X + this.commandBarDropDownButton2.Size.Width, eventArgs.CustomLocation.Y);
        }
    }
}