I noticed that the last 4 digits of the card used with Apple Pay is different every time I pay (I use my Apple Watch mostly). Not use between merchants but also with the same merchant.
Are you sure you're not mixing up phone and watch payments? And is this with a Mastercard or Visa card, or something region-specific?
The DPAN has always been constant in many years of using Apple Pay for me, which is useful to e.g. check which card I've actually used on by comparing the last four digits printed on a receipt to those I can see on my phone or watch (the card details show both the funding and the DPAN last four digits).
