If the billing address reverts back to your previous address then the issue is probably related to the Contacts app. There is an old thread that I started about 6 or 7 years ago about the same issue which plagued many people. We had recently moved to a new address and the billing address in Apple Pay constantly reverted back to our previous address despite being changed multiple times.
My solution at that time was to get my bank, it was my debit card from them with the problem, to issue me a new card with a different account number. When that card was activated and entered into Apple Pay the correct address, which my bank already had, was picked up and retained. However someone else came upon the Contacts app as the problem. So delete your Contact, the “Me” one, and then create a new one with all your current, correct info, and set it as your “Me” card. Apple Pay should then pick that up for your billing address. This is apparently some kind of corruption with the Contacts data base that just hangs onto your old address for dear life. My way, getting a new card with a new account number, did work but it is a drastic move to make. Try the contact card change first.