Sign in with Apple fails with ASAuthorizationError.canceled (1001) / "Sign Up Not Completed" — server-side, all config verified

Our app's Sign in with Apple never completes. Apple's create-account sheet renders fully (name + share/hide-my-email step), then the final server submit fails and shows the "Sign Up Not Completed" sheet. The authorization callback returns ASAuthorizationError.canceled (1001) with no userInfo — even though the user did not cancel. Reproduced on multiple physical devices and multiple Apple IDs (all with 2FA), on iOS 26.5.1, since our first build.

• Team ID: 24XS837728
• App ID: C54N9Q226H (bundle com.sucsessmaperrs.careerAssessmentApp)

Verified on our side:

• com.apple.developer.applesignin = [Default] present in the distribution build (codesign -d --entitlements); embedded profile is the

explicit App Store profile for the App ID (not wildcard).

• App ID has Sign in with Apple enabled as a primary App ID; capability deleted + recreated via the App Store Connect API today, no

change.

• Firebase consumes the identity token; the failure is in Apple's UI before our code runs.
• App Store Connect agreements are clean.

Key isolating fact: the same Apple ID on the same device completes a first-time Sign in with Apple sign-up in other third-party apps — only ours fails. So the cause appears scoped to our App ID / team on Apple's side, not the device, the Apple ID, 2FA, or our client code.

This matches the widespread server-side "Sign Up Not Completed" failure in thread 675756 — same signature: multiple devices/Apple IDs, no console error, the failure delegate surfacing only as "cancel", newly created apps affected, even Apple's own sample app failing — which Apple ultimately resolved server-side (with a recurrence reported June 2025). Note this is the native ASAuthorizationController flow, not the REST/web flow, so the query-parameter percent-encoding fix from thread 122458 does not apply here.

Could an Apple engineer check the server-side Sign in with Apple / account-association state for App ID C54N9Q226H (Team 24XS837728)? Happy to provide a screen recording or sysdiagnose.