Relevant passage from https://developer.apple.com/documentation/sign_in_with_apple/sign_in_with_apple_rest_api/authenticating_users_with_sign_in_with_apple:
"Although Apple provides the user’s email address in the identity token on all subsequent API responses, it doesn’t include other information about the user, such as their name. When you receive user information from the API response, immediately store it locally so your app can access it again in the event of a process or network failure."