Hi Travis, thanks for picking this up.
Sample project: done — a minimal reproducer (GCMultiUserRepro.zip) is now attached to FB24156316, along with a fresh sysdiagnose captured immediately after reproducing with it today (tvOS 26.5, Apple TV 4K), and screenshots. The report also has the full log analysis, including one new finding: Settings flagged the secondary user's account with "Some iCloud data isn't syncing" and offered the "Update Your Apple Account" proximity flow — completing it clears the warning but doesn't fix matchmaking.
Is it limited to Apple TV? As far as I can test, yes. The trigger is specifically a non-default TV user — a precondition iOS doesn't have — and it reproduces in every GC app I tried (including Apple Arcade's Crossy Road Castle), in both GC environments, with two unrelated Apple Accounts. The same codebase plays Game Center matches fine on iPadOS 26.
That said, the failing step might interest you for the iOS report: gamed's pseudonym provisioning finds no identityservicesd registration for the secondary user's Apple Account ("No URI found on any account" → IDS 400), and the app receives GKError 35 "not signed in to iCloud" even though iCloud sign-in is healthy. If your iOS report shows that same signature, it may be the same missing-IDS-registration dependency reached by a different route.
