Filed as FB23707746 with a self-contained repro. The new detail: the app never builds a mixed queue. Hand ApplicationMusicPlayer a pure catalog Song array and the system substitutes the local library item for any catalog id the user owns, then fails on the mix it created (prepareToPlay → MPMusicPlayerControllerErrorDomain code 6, or "Prepare queue failed with unexpected start item" when the start item is the library-matched song). Queue(for:) without a start item is worse! It "succeeds" while silently discarding every store-only song. Same defect as FB10967343 (July 2022), still unfixed on macOS 26.5.2.