Following up on this issue. I can say that we’re seeing a large number of these cases coming from Australian/NZ stores which makes me suspect the checkout providers they use?
When we pass the discount_code as a query parameter to the checkout endpoint we get in the return headers a set-cookie header of discount_code with the value as expected but then when the page is redirected with a 302 status the designated checkout page sends a set-cookie header of discount_code=; which removes the discount.
Any advice will help here Shopify. Thanks