Yeah, I'm having this specific problem. Our app is being used on locations with bad internet connection, and I'm already expecting people stating that they cannot signin right the very first time, and then we won't have their email, which is mandatory to create an account on our system.
weirdly enough even if I logout of iCloud, or remove the app, it doesn't matter. since the very next time they try to signin with Apple, if on the first attempt was unsuccessful, and we havent got their email, we won't be able to register again.
Question to Apple: what is more secure, sending the email / name (depending on the scopes) everytime we request it , or asking the users to "securely cache the credentials" which will leave breadcrumbs of the users identity somehow/somewhere (DB, keychain, some other store...) which probably could be accessable by other applications ?
Another relaeted quick question, if the user used signed in properly with their actual email, but then decide they want to revoke that and use one of the private emails Apple provides, is it possible ? Or after you allow it once , that's it ? It's possible on https://appleid.apple.com/account/manage