Please refer to the attached gif file.

Workaround:  cancel the PreviewPan event and adjust manually the  Telerik.WinControls.UI.Diagrams.Panel.PositionOffset. Here is demonstrated a sample approach:
int widthOffset = -1000;
int heightOffset = -1000;

private void radDiagram_PreviewPan(object sender, Telerik.WinControls.UI.Diagrams.PositionChangedRoutedEventArgs e)
{
    Telerik.WinControls.UI.Diagrams.Panel panel = this.radDiagram.DiagramElement.Children.First() as Telerik.WinControls.UI.Diagrams.Panel;
    Telerik.WinControls.UI.Diagrams.Panel hitTestablePanel = panel.Children.First() as Telerik.WinControls.UI.Diagrams.Panel;

    e.Cancel = true;
    if (panel != null)
    {
        hitTestablePanel.PositionOffset = new SizeF(widthOffset,heightOffset);
        widthOffset += 50;
    }
}