If you have 'Automatic Downloads' enabled in iTunes, which pre-downloads any new tracks you add to your Apple Music library (as opposed to streaming them), a similar thing happens if you're subscribed to any playlists that update frequently.
For example, Apple themselves have a stack of playlists for different genres that are updated on a weekly basis, so after a few months you end up with a ton of downloaded files for songs that are no longer in your library (because they've been rotated out of the playlists).
Problem is, because they're not in your library there's nothing surfaced in the UI, so there's actually no way to know that it doesn't clean up after itself until you go to investigate why you have no disk space.
No idea if this behaviour has survived the transition to Music.app, but considering it seems to just be iTunes with a fresh coat of paint, it wouldn't surprise me.
For example, Apple themselves have a stack of playlists for different genres that are updated on a weekly basis, so after a few months you end up with a ton of downloaded files for songs that are no longer in your library (because they've been rotated out of the playlists).
Problem is, because they're not in your library there's nothing surfaced in the UI, so there's actually no way to know that it doesn't clean up after itself until you go to investigate why you have no disk space.
No idea if this behaviour has survived the transition to Music.app, but considering it seems to just be iTunes with a fresh coat of paint, it wouldn't surprise me.