The issue I received when tapping to pay was "Your phone doesn't meet software standards". It did mention it can be due to rooting (my phone wasn't rooted) or "uncertified software" (of which I didn't have any).

I'm sure you might already do so, but I'd advise not to rely on the phone to pay going forward. For me, I was caught off guard in a shop without any other payment method. There was no warning or notification about the change until I tapped to pay at the card terminal.

I tried a few more times afterwards with another payment method as backup, and it never worked again, even after rebooting, toggling NFC off and on. I never received an email, notification or warning in the app itself. It was quite disappointing as I reckon I could have run that phone for a few more years as a minimal phone.

Google did offer me about £100 off of a new Pixel. So if this happens to you and the lack of Google Pay is a dealbreaker, I'd give that a try.