Thanks - assuming you have been in the USA from when it last worked until now, then it can’t be some crazy geo-location issue because the physical location hasn’t changed.
So it’s either an app-version issue (I am sure the iOS app has changed in the last few weeks) and/or a deeper issue with multiple account types. This is a very unique use-case and one that Monzo may not be aware of. I’d recommend reporting this issue to Monzo via in-app chat and I’m sure it’ll be escalated to a specialist. Just be sure to inform you have both US and UK accounts, have the US-sourced version of the app (provide the app version), your location and that the UK ‘blue pill’ account display did work up until a few weeks ago.
Good luck! 