I think it’s just that it could happen with any card that does online transactions (don’t know how many others there are now) but Monzo is a bright colour, so people see it and think “Unusual colour - must be a dodgy card”. All that needs to happen is if someone walks away while the card machine still says it’s processing, before it says “Approved”, the vendor says “One moment please, that’s still processing”. Problem solved. But people don’t realise that and jump to conclusions. Also sometimes someone spreads a misinformed rumour like “Monzo users can cancel payments afterwards so you don’t get your money” and people panic and believe it.
I just use Android pay all the time now anyway (unless they’re on an old machine that won’t let me for over £30) and I find it hilarious that vendors won’t even know if you’re using Monzo that way
I’ve not encountered this issue anywhere still, but if I did I’d definitely have a chat and try to address misconceptions, and if they were stubborn about it would mention that they state they accept Mastercard and Mastercard might not be happy if they are refusing to take certain ones.
I did have an issue once in Tesco where they authorised the payment but the terminal said it failed. I had to pay again but both payments were authorised on my account. They don’t actually take the money for a few days so it would disappear at that point, but Monzo allows you to cancel it immediately if you wish in the app, with the caveat that you’ll pay if the company does turn around and charge you. Maybe it was something similar with pret, just that they cancelled the authorisation straight away? I guess with my legacy bank it would never have even showed up probably so I wouldn’t have known.