Discounting human error early in any diagnosis is usually a good move.
There is a particular self-serve checkout terminal in a particular Tesco store near us which won’t accept the Monzo Joint account via NFC/Gpay. We’ve tested this: Other cards via NFC/Gpay work fine, as do both physical Joint account cards. And the Joint account works with other checkout terminals in the same store via NFC/Gpay.
Try explaining that one to the Tesco staff - ‘contact your bank’ is the only response. It’s got nothing to do with the bank but we’ve done the acceptance@monzo.com route anyway.
There are lots of variables involved with something like this. Remember the Apple Pay issue with Sumup terminals earlier this year?:
I’d recommend sending the details of the failure to acceptance@monzo.com