Prepaid card went many years ago.
This appears to be caused by being the beta version of the app initially, then just updating it each time, I don’t recall ever uninstalling it until now. (I’ve just highlighted that in my post.)
I imagine Monzo closed the beta their side and Google Play left me on it until I actually uninstalled the app - rather than just updating. AFAIK there is no longer an option to re-join it.
It appears the sequence to leave the beta is important.
TestFlight may well be the correct term!!
Edit: It looks like TestFlight is the term Apple uses, not Google / Android / Play.
For Apple Yes, I don’t think the “beta” you’ve linked is the same thing. That’s the OG Beta, when it was a pre-paid card.
I’ve never known this issue persist on Apple, must be that inferior android 
(with apologies to @davidwalton).
Oh! This isn’t the same as the threads you’ve been replying to. It’s not the Beta account you’re referring to, but the TestFlight app. Completely different.
None taken.
How different is this on Android? I suspect this is a one off bug. On TestFlight you install the TestFlight version which replaces the App Store version. To “leave” you just delete the Testaflight and reinstall the App Store version.
On Android you just opt-in to the beta and the app updates to the beta version. Opt back out and the app will revert back to the regular version.