Apple Pay button not appearing

On an older iPhone I'm testing with (6s, iOS 12.5.7), connected to the same Sandbox Apple ID that I'm using on multiple other devices, the Apple Pay button is not appearing. Neither on my web application, whatever version I set in the PaymentRequest, nor on the official demo site.

Further, 2 sandbox cards that were added fine to these other devices are failing to add on this one. Same cards and CVV codes are getting errors. At least "Invalid Card" on one of them. Although the other failed several times, then just worked this time when I tried it again 😕

But on this phone, I have two cards successfully added, so the button should be appearing. On the demo site, whether I select Apple Pay JS API or Payment Request API, the button does not appear.