This is late now but maybe it'll help others that stumble onto this page like me.
What you are supposed to do in this case is saving your user's email on first successful sign-in with Apple to the Keychain, and not in the app's storage.
That way, when the user reinstalls your app and the first attempt to sign in with Apple succeeds but returns no credentials, you know you can use the ones stored in the Keychain.
Hope it helps someone.
Jeannustre
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