Why are my receipts showing a different card number when using Apple Pay?
When your debit card is added to Apple Pay, it creates a unique Device Account Number. The last 4 digits of this number are shown on receipts in place of your card number. This is part of Apple Pay's built-in security measures that help protect your data during payment transactions.
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.