I believe sometimes a song become unavailable for some reason (license issues or whatever) and they replace it with a similar version. Most times it's just the exact same song in a different album, some cases it's a different recording from the same artist, but in the rare cases the algorithm may find a cover version to offer as a suitable replacement. I think the main issue is they don't keep track of the original song that received the like, so when it comes back, it's not restored to your library.