Use attached to reproduce.
Workaround:
private void RadDiagram1_ItemsChanged(object sender, Telerik.WinControls.UI.Diagrams.DiagramItemsChangedEventArgs e)
{
if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
{
IEditableObject editableObject = ((RadDiagramShape)e.NewItems.First()).DataBoundItem as IEditableObject;
if (editableObject != null)
{
editableObject.EndEdit();
}
}
}