I made an order with quantity of 6 in the development store from the Shopify partners panel, then I returned the 3. Everything's visible in the panel, but none of this information about returned items (their quantity,return dates, etc) gets carried over to the response of the API call, no field related to returns, no change in the fulfillments attribute of the order API call after the return changes. Nothing I could find in the API reference.
Thanks for getting in touch. Please could I obtain an x-request-id that would be returned the the call headers when you made the API call to the Orders.json endpoint when creating the order? I'll take a look into it. Thanks!