This was probably intentional, they don’t really need to say that somebody unlike or undisliked something. You just need to know that their vote is no longer valid. This is probably easier with Mastodon as well because they can and just erase whatever about it was, plus one or minus one, and essentially they don’t have a vote anymore.
Because if you really think about it, do you really need to know if somebody undisliked versus unlike something? Or do you only need to know that their vote is now removed?