Workaround: suspend columns notifications when preview dropping and resume the notification after the drag drop service has stopped:

Dim svc As RadDragDropService = Me.RadGridView1.GridViewElement.GetService(Of RadDragDropService)()
AddHandler svc.Stopped, svc_Stopped
AddHandler svc.PreviewDragDrop, AddressOf svc_PreviewDragDrop

Private Sub svc_PreviewDragDrop(sender As Object, e As RadDropEventArgs)
    For Each col As GridViewColumn In Me.RadGridView1.Columns
        col.SuspendPropertyNotifications()
    Next
End Sub

Private Function svc_Stopped() As Object
    For Each col As GridViewColumn In Me.RadGridView1.Columns
        col.ResumePropertyNotifications()
    Next
End Function