Firebase phone auth OTP no longer being read automatically

Android device: Any device
Android OS version: Any version (have tested this starting from OS21 all the way to the latest OS - bug repros everywhere)
Google Play Services version: com.google.gms:google-services:4.3.8
Firebase/Play Services SDK version: com.google.firebase:firebase-auth:21.0.1
FirebaseUI version: com.firebaseui:firebase-ui-auth:7.1.1

Steps to reproduce:
OTP was being read automatically in my app for many months now. It has suddenly stopped working in the last few weeks. There are no specific steps to reproduce - OTP is never read automatically anymore. My code (which has been working for many months now) has not changed at all - it is pretty much the same as the code in the Firebase Auth documentation / samples - nothing special here.

I have verified that the SMS template being used is correct and matches the OTP received. I have also enabled Android SafetyNet / app verification, to no avail.

I filed an issue in the Firebase Auth UI repo (https://github.com/firebase/FirebaseUI-Android/issues/1948) and was directed to file an issue here instead.