When logging into an account that uses SMS-based 2FA, Proton Pass should automatically detect the incoming OTP and offer to autofill it with a single tap — just like it already does for TOTP codes.

This could be done using Android's SMS Retriever API, similar to what was implemented by Dahslane and other password managers.

Reference
https://www.dashlane.com/blog/dashlane-android-retrieve-sms-codes
https://developers.google.com/identity/sms-retriever/overview