When a card is declined, your bank's reason is the useful one, and checkout still shows it to you exactly as it arrives.
What changed is everything else. Some payment failures are not about your card at all - they are a fault at our end, and until now checkout would put the raw technical message on screen and leave it there. It read like you had done something wrong, it mentioned things that mean nothing outside our codebase, and worst of all nobody here found out. The card form talks to our payment provider directly from your browser, so a failure of that kind never reached our records. Checkout could have been broken for everyone and our logs would have looked like a normal quiet evening.
Now every failed payment is written down, and the ones that are our fault send us an alert immediately. If you hit one, you will see a short message telling you your card has not been charged and that we already know. Repeated reports of the same fault are grouped, so a problem affecting lots of people reaches us as one clear alarm rather than thousands of separate ones.
Nothing about your card is included in any of this. The card number never touches our servers at any point, and the reports carry only the payment provider's description of what went wrong.