It looks like it’s taking my cards listing endpoint a few seconds to refresh the data after adding / deleting a card in the sandbox.
I’ll delete a card, then I can make 2 - 3 api calls before it actually disappears from the endpoint.
Similar results when adding a card.
Is anyone else experiencing this issue, as if the cards endpoint is being cached?
After successfully adding or deleting cards there may be a small delay before you’re able to retrieve the information from the ListCards endpoint if you are calling it right after you added or removed the card. We recommend either waiting a bit to pull the data or create a loop until you’re able to successfully retrieve the card details. 
Thanks Bryan!
This essentially means a headless approach isn’t viable for the api - I should store the card reference locally and delete it from my database on a successful delete endpoint response.
This is OK, but personally I think it would a huge benefit to consumers of the api if we could implement a headless approach, reducing work load, complexity and security concerns in the time spent building implementations. Perhaps the team could consider invalidating the cache when a change is made to a user’s cards 
Many thanks for your reply!
We’re constantly working to improve our features based on feedback like this, so I’ll be sure to share your request to the API product team.