Unauthorised Payment Card Appears in Safari Autofill

I have discovered an unknown credit card in my Safari Keychain on my multiple devices. It's a China UnionPay Card.

There are at least three threads on this site asking questions about this same problem, the most recent with posts from late last year. The thread with this title is the only one with meaningful, if not necessarily correct, answers.

I've rung Apple, and the support person was unaware of this issue, but just suggested I delete the card entry from my Keychain, which I've done, and change my Keychain password (likewise).

The previous threads have either had no suggestions, or said that it was related to Amex or Ikea cards. I have neither of these cards. Apple suggested I look for transactions at the time of the new entry (about a month ago), but I cannot find any in my bank transactions or e-mails.

Any fresh ideas, please?