Telerik Diagramming Framework supports UI Virtualization, which collapses all items outside the viewable area, thus enhancing the UI performance while manipulating the Diagramming elements. However as the virtualization implementation only collapses the visibility of the controls outside the veiwport, those items will still be considered during a layout operation for example. This means that a Layout operation will change the visibility of the Diagramming elements if they are brought into the viewport area.
By default the Virtualization feature of the RadDiagram is enabled. You can disable it by setting the DiagramSurface.IsVirtualizing attached property to False.
<telerik:RadDiagram x:Name="diagram" telerik:DiagramSurface.IsVirtualizing="False"/>