Hey blake05,
I was able to repro your issue successfully. Unfortunately, Delta<T> was tested with only the ODataFormatter. We never tried Delta<T> with the json formatter. Some of the things might be working with the inbuilt json formatter as Delta<T> is also a DynamicObject.
Have you considered using the ODataMediaTypeFormatter ? I know there is more work in exposing an OData service and opting into the odata format.