I filed FB13689023 for this since it is clearly unexpected.
Non Apple Music library tracks are not returned on macOS using MusicLibraryRequest. I tried querying by artist where I only get Apple Music tracks, and by id where I don't get the track.
On iPhone and iPad I was able to get both AM and non-AM tracks, that's how I got a valid id to try on macOS. Sync Library is on.
The build targets My Mac (Designed for iPad).
I'm using Xcode 15.3 and I'm on Sonoma (14.4).
Any help is appreciated...