Online purchase with Apple Pay shows actual last 4 digits of credit card

There is a discussion of this issue here, but no answer was ever given: Why does my Apple Pay receipt show the ac… - Apple Community

I made an online purchase using Apple Pay in Safari (and had an infuriating user experience, but that’s a separate issue). Imagine that my actual card number ends with 1234. On the purchase confirmation screen from the vendor, it shows the payment method as:

[Apple Pay icon] Amex **** 1234

Can someone please explain why this is happening? Transmitting any part of my actual card number defeats the purpose of Apple Pay.

Mac OS 15.2

Safari 18.2