Ever had someone "patch" an album you already bought, where you still have persistent download permissions on the storefront/library but now they point to a different version of the music?
I have the Baba is You soundtrack on Steam, and at some point it added five new tracks at the end! This would have to have happened after the "baba make level" / second quest update came out, since they seem to be tracks from those game additions. I don't even know why it occurred to me to check, it didn't notify me or anything.
Anyway, yay for that, I downloaded em. But anyway, this reminds me of the first time I found a picture hidden behind the (opaque) CD tray — a new thing to watch for.
(IMO this is distinct from the mutation/rugpull/etc. of music on streaming services.)