As mentioned, this already exists, and has for as long as I can remember.
I use biometrics to login to ProtonMail all the time. I use long random passwords for all my logins, and without the biometrics, it would be a major hassle for me to login every time. I'd have to open my password manager, copy the password, and paste it into the password field every time. I use Enpass for some of my passwords, and Proton Pass for others, and Proton Pass uses the same password as Proton Mail, I still need Enpass to remember the password without biometrics.

They have biometrics in the android app. Try the following in app:
- settings
- auto lock
- options for PIN lock and for biometrics

Agreed. Another point is in case your device is used by an unauthorised third party. Tutanota also has the option of requiring either system password or biometrics. For those who don't use biometrics, that would be a fine alternative.

I'm not personally a fan of biometrics for application authn so if you implement this I won't be using it personally - but there are others who do like the idea. That having been said if that is something being considered look at Truststamp's fully encrypted, irreversible, biometric tokenization: https://truststamp.ai

"Technology that creates unique, irreversible hashed-tokenized identities and facilitates secure, interoperable identity proofing – anywhere in the world, online and offline."

Caveat: the OP is attempting to solve a different problem than biometric/authn - they simply want the logout to stop on app update - which is entirely unrelated to authn mechanisms. So my concurrence here is to the matter of biometric/authn in general rather than their specific complaint of "auto-logout on update" for which I can envision quite a number of update situations in which logging out would be required.