The default implementation of the Drop event handler of the diagram handles the event (sets the e.Handled property of the arguments to True). This is why the OnDiagramDrop() event handler is never executed. In order to resolve this you can set the last argument (handledEventsToo parameter) of the DragDropManager.AddDropHandler() method to True.
This will ensure that the event will be executed even if is handled.
Please try this and let me know if it helps.
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