With Apple Pay transactions your receipts do not show the last few digits of the actual credit card. They instead show the last few digits of the Device Account Number. This is the normal function of Apple Pay and it is extremely secure for the user in terms of limiting their card number's exposure.
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.
It is recommended when processing a refund to ask the customer to show you the last few digits of this number on their 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.