I think all users received mail from monzo for updating app and changing pin as security reason. I have got the updated app via this community, but I could not change pin as I have no plan landing on UK recently. I live in China, I use a mastercard ATM but there is no option for pin whilst only cards issued by bank of ATM can have that option. How do you change your pin if you are also out of UK?
Wait until you get back. That is what Monzo suggests in your situation.
In my opinion, it really isn’t a major or even minor security issue. Monzo are just being super honest and legally have to say what they said. In the end, the chances of fraud are almost zero and Monzo has your back if there is fraud based on the possibility a very few engineers who currently work for Monzo (but may leave) attempting to commit fraud against their own (or future former) employer after possibly, but not even certainly, they have seen your specific PIN number recorded in encrypted log files.
I have not changed my PIN and I live less than 20 metres away from 2 ATM’s.