Shipping rates generate-shipping-rates endpoint only returns subscription shipping results?

Using the cart API to get shipping rates at the cart, but if there is a subscription in the cart then it only seems to return the subscription portion of the shipping rate and not the one-off product shipping rate? Am I missing something or is there a way around this?

We’re experiencing a similar problem with this API, I’ve raised a separate issue here: https://community.shopify.com/post/2153167