Use attached to reproduce.

Workaround:

AddHandler RadDropDownList2.DropDownListElement.AutoCompleteSuggest.DropDownList.PopupOpening, AddressOf RadDropDownList2_PopupOpening
RadDropDownList2.DropDownListElement.AutoCompleteSuggest.DropDownList.MaxDropDownItems = 10

Private Sub RadDropDownList2_PopupOpening(sender As Object, e As System.ComponentModel.CancelEventArgs)
    Dim args = DirectCast(e, RadPopupOpeningEventArgs)

    Dim ownerElement = (TryCast(sender, RadDropDownListElement))
    Dim loc = ownerElement.ElementTree.Control.PointToScreen(ownerElement.ControlBoundingRectangle.Location)


    args.CustomLocation = New Point(loc.X, loc.Y + RadDropDownList2.Height)
End Sub