Funny, to me this sounds like a hash collision. (For a map key, not the cryptographic kind.) Knowing only that Spotify is a large distributed system, and seeing that the mystery tracks are short in title and length, as well as plentiful, it feels like a these tracks are ending up as collisions in a distributed hash table. From there it'd be a matter of bubbling up top due to sorting order properties.
Is that something that is done in some industries? Hash collisions to me has always been a performance problem not a correctness one.