It’s important to note that this PIN/Password/Fingerprint is an app lock mechanism only, this PIN should ideally be different to your card’s PIN. Since it’s an app-related PIN; I wouldn’t expect it to be there when reinstalling app, so I don’t think there really needs to be a recovery method. If someone forgets it, then they can just be permanently locked out requiring them to go through email re-authentication with magic link again after which they can then set a PIN as if for the first time on a fresh app.
More than a fallback I think the reasoning in Android is that fingerprint is never the main method of unlocking the phone, it’s just an extra that simplifies the actual unlocking process. Fingerprint has its own entry in the security settings (at least on my phone, I can provide screenshots if needed) and they are not included in the main lock options of which you need to choose one before you can even enable fingerprint. You also can’t use fingerprint to unlock phone on start-up for some reason. This is the kind of fingerprint authentication philosophy I would expect: PIN first and a fingerprint if phone-enabled to bypass this in a faster, handier way.
I personally don’t care for this extra security, my phone is pin/fingerprint protected and I’m its only user. But I do feel that if Monzo are going to allow users to lock their apps, they should allow a PIN fallback because while it doesn’t happen regularly; every now and again, my fingers don’t work and I have to unlock phone with PIN. It would frustrate the life out of me to occasionally find myself locked out of the Monzo app because fingerprint wasn’t being recognised.