Workaround: 

this.radTextBoxControl1.TextBoxElement.AutoCompleteDropDown.PopupOpening += AutoCompleteDropDown_PopupOpening;

private void AutoCompleteDropDown_PopupOpening(object sender, CancelEventArgs args)
{
    RadPopupOpeningEventArgs a = args as RadPopupOpeningEventArgs;
    if (a != null )
    {
        a.CustomLocation = new Point(a.CustomLocation.X,a.CustomLocation.Y + 20);
    }
}