Hi Dave
Your receipts are not supposed to show the last few digits of your actual credit card. They instead show the last few digits of the Device Account Number.
Whenever a payment card is added to Apple Pay, the combination of that card and that device (Apple Watch, iPhone, iPad or Mac) is given a unique Device Account Number.
Using this number instead of your actual card number forms part of Apple Pay's built-in security measures that help to protect your data during payment transactions.
When returning an item, you may be asked to show the last few digits of this number on your device.
To view the last few digits of the Device Account Number on the device that was used for payment:
On iPhone: Open Wallet > select the card that was used for payment > tap on the info "i" icon at the lower-right > tap on the Info tab > the last few digits of your Device Account Number are displayed under Card Information.
For payments made via Apple Watch: on your iPhone, in the Watch app, go to: My Watch > Wallet & Apple Pay > tap on the payment card > the last few digits of your Device Account Number are displayed under Card Information.
More information about why this feature is used - along with information about Apple Pay's other privacy and security features - is available here:
Apple Pay security and privacy overview - Apple Support