Monzo doesn’t natively support the resolution of the 12 phones.
For Apps like that Apple scales the XS/X resolution and zooms in so it fills the larger display (think of it like the zoomed mode option you get when setting up the new phone, but less noticeable and less intense). This results in the app using a slightly larger text size than default and in apps that natively support the correct resolution. It was either do this, or have black boarders until apps were updated to support the larger dimensions!
That’s why it’s cut off for @monzo2018 (happy cake day btw!) and not for you until you increased your text size.
It’s something I actually noticed a while back and reported on here: