That feature is the single worst thing Apple has done I have ever encountered: it stole my music library.
At some point when migrating from one Mac to another, it "forgot" which songs were actually mine. It's all Apple Music now. I have songs that the application _knows_ were added to my library in 2003, but for which it steadfastly maintains they're Apple Music downloads. Worse: some songs have been replaced with other recordings. Other are "unavailable" for unexplained reasons.
Oof, I would be livid if that happened to me, too. Maybe I just threaded the needle and got lucky or something, but I'm disappointed to hear mine isn't a universal experience.
At some point when migrating from one Mac to another, it "forgot" which songs were actually mine. It's all Apple Music now. I have songs that the application _knows_ were added to my library in 2003, but for which it steadfastly maintains they're Apple Music downloads. Worse: some songs have been replaced with other recordings. Other are "unavailable" for unexplained reasons.