To reproduce:
- Run the attached project, maximize the form and click the drop-down buttons.

Workaround:
commandBarDropDownButton3.DropDownMenu.PopupOpening += DropDownMenu_PopupOpening;


private void DropDownMenu_PopupOpening(object sender, CancelEventArgs args)
{
    var e = args as RadPopupOpeningEventArgs;
    var ownerElement = (sender as RadDropDownMenu).OwnerElement;
    var loc = ownerElement.ElementTree.Control.PointToScreen(ownerElement.ControlBoundingRectangle.Location);
    loc.Y += ownerElement.ControlBoundingRectangle.Height;
    loc.X -=  ((sender as RadDropDownMenu).Width - ownerElement.ControlBoundingRectangle.Width);
    e.CustomLocation = loc;

}