We are hitting the same issue. Team ID 7XWPRV3856, App IDs com.fixli.user.me and com.fixli.me, both registered 2026-08-18. Native sheet authenticates with Face ID, then "Sign Up Not Completed" / ASAuthorizationError.canceled (1001) with empty userInfo, on TestFlight builds on physical devices. Entitlement com.apple.developer.applesignin verified present in the signed binaries and embedded provisioning profiles; PRIMARY_APP_CONSENT confirmed via the App Store Connect API; capability re-registered and both App Store profiles regenerated on 2026-08-25 with no change. Fails identically with Share My Email and Hide My Email. Filed as FB24504006 (screen recording attached; sysdiagnose with the Accounts/AuthKit profile to follow — the affected devices are with remote testers). Requesting the same server-side registration repair applied to the other teams in this thread.