Some users cannot use Authenticator OTP

After restricting insecure MFA methods a month ago, we sporadically get users complaining about the authenticator app. In the most recent case, the user only has the option to receive a push notification. This is a problem for users where the Internet is temporarily unavailable. These users used to have the option, and we allow OTP in Azure. We're targeting all users, so there's no reason specific users be affected. The usual fix is re-registering their authenticator app.