Due to what I assume are music licensing oddities, one song in my Spotify now has an entirely different singer. It was taken down for a while, then later reuploaded as a new recording with new vocals. I’ve also seen copies of my songs changing to remix/cover versions, presumably due to metadata adjustments by the artists. Although usually I can re-search and find the original, adding it back to my library.
A few songs just have new outros/intros. One song in my library now has an additional several seconds of silence on the end. One has a new longer intro that I think is really from the music video version of the song.
As convenient as these music services are, I hate that my library changes beneath me without my control. There are dozens of songs just straight up missing from my Spotify library now. And a small handful that have changed audio. These are almost always in indie songs, and Spotify just hides deleted songs by default in the UI so most users don’t notice.