I am having the same issue with the model binding when attempting to bind a collection of GraphLinksModel to a TabControl. The diagram won’t update its model when inside the TabControl.ContentTemplate, even though the binding is correct. I took the exact same XAML I used in this DataTemplate and bound to the first item in the collection and it works fine. Why is there an issue when the diagram is placed directly in a DataTemplate?
Are you binding Diagram.Model?
If so, you may need to set Diagram.HasDefaultModel to false. HasDefaultModel Property
I tried setting Diagram.HasDefaultModel, but then it broke the Model binding when the diagram is in a UserControl.