Workaround:

private void RadDropDownButton1_DropDownOpening(object sender, EventArgs e)
{
    var args = e as RadPopupOpeningEventArgs;
    var popupSize = radDropDownButton1.DropDownButtonElement.DropDownMenu.Size;
    var x = args.CustomLocation.X - (popupSize.Width - radDropDownButton1.Width);
    args.CustomLocation = new Point(x, args.CustomLocation.Y);
}