Sign in with Apple fails with "Sign-Up Not Completed" for one App ID only — other apps in the same team work on the same device

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Sign in with Apple fails with "Sign-Up Not Completed" for one App ID only — other apps in the same team work on the same device

I have a Sign in with Apple failure that is isolated to a single App ID, and I would appreciate an Apple engineer looking at the server-side registration state for it. Filed as FB24483427.

Symptom

For App ID com.laonmon.twothrones (Team LH4XJ8LL98), Sign in with Apple never completes:

The native ASAuthorizationController sheet appears correctly.
The user authenticates with Face ID / Touch ID.
iOS shows "Sign-Up Not Completed".
The delegate receives ASAuthorizationError code 1000 with no further detail.

No request ever reaches our server, so the failure happens entirely inside Apple's authorization flow. Our production database shows 0 accounts created via Sign in with Apple out of 147 total accounts — it has never succeeded for any user.

The control experiment

This is the part I think is diagnostic. Tested in one sitting, on the same iPad, signed in with the same standard (non-managed, adult) Apple ID:

Notion (third-party App Store app) — sign-up completes
SwingNote (my own released app, com.swingnote.app, Team LH4XJ8LL98) — sign-up completes
Two Thrones (com.laonmon.twothrones, Team LH4XJ8LL98) — "Sign-Up Not Completed"

Same device, same Apple ID, same team, same capability configuration — only the App ID differs. That rules out the device, the Apple ID, the team account, and the local configuration.

What I have already checked

Entitlement is in the signed binary. codesign -d --entitlements - on the archive shows com.apple.developer.applesignin = ["Default"]. (I did find and fix a real build defect earlier: our archive step ran with CODE_SIGNING_ALLOWED=NO, which skips entitlement processing entirely. Before that fix the sheet did not appear at all. Worth checking if you see no sheet — but that is a different symptom from this one.)
App ID capability is enabled — APPLE_ID_AUTH, APPLE_ID_AUTH_APP_CONSENT = PRIMARY_APP_CONSENT. I removed and re-added the capability and regenerated the provisioning profile. No change.
All four App IDs in the team have identical Sign in with Apple settings.
Settings › Sign in with Apple does not list the app on the test device, so no app-scoped identifier was ever created. The sign-up genuinely never completes rather than failing on reuse.
Updated Apple Developer Program License Agreement accepted; App Store Connect banner cleared.
This is not propagation delay. I have seen the thread where it started working after ~2 days. The capability has been configured since Aug 16 and has never succeeded — across three App Review sessions on different devices (iPad Air 11-inch M3 / iPadOS 26.6, iPhone 17 Pro Max / iOS 26.6) and my own testing.
Device logs captured over USB show AuthenticationServicesUI and AuthKitUIService starting normally, but all authentication detail is redacted as <private>. I understand the Accounts/AuthKit logging profile is needed to read it, and I will attach a sysdiagnose to the Feedback report once I have it.

Impact

Three consecutive App Review rejections under Guideline 2.1(a). This is the only thing blocking our initial release.

Question

Given that the same device and Apple ID succeed for another app in the same team, is there a server-side Sign in with Apple registration or account-association state per App ID that can get into a bad state, and is there anything a developer can do to reset it? Anything I can check or provide would be welcome.

Feedback: FB24483427 DTS case: 20000144146252 (Developer Support advised filing here — they cannot see Feedback Assistant reports themselves.)