On macOS 27.0 seed 6 (26A5421a) / Xcode 27 beta 6, the first touch of AssetPackManager.shared in a Managed Background Assets macOS app fatals at AssetPackManager.swift:360 with The app couldn't be validated: The app group with the ID "TEAMID.group.example.app" is inaccessible — with the app signed with exactly that group, BAAppGroupID matching, containerURL(forSecurityApplicationGroupIdentifier:) returning a real directory, and the container present on disk.
I tried varying it and got the same result: App Sandbox on/off; the group spelled team-prefixed and iOS-style at every point of use. Notably, in the iOS-style build the message still names the team-prefixed identifier, which appears nowhere in that build — so the framework composes the prefixed form itself and validates that. The Mac provisioning profile authorizes group.example.app explicitly plus the auto-added TEAMID.* wildcard, and since the portal only registers group.-style IDs, no profile can list the prefixed form by name.
Is this a known issue in seed 6? Is there a workaround? And does Managed Background Assets on macOS assume a Mac App Store / TestFlight distribution context for its app-group validation, in a way a development-signed app can't satisfy?
Filed as FB24525451 with three crash reports (sandboxed, unsandboxed, iOS-style) and a sysdiagnose.