Normally, if I refresh a page once and immediately vote, it works. In this case, it has never worked.
This happens periodically and it does not seem to be specific to any instance (I've seen across posts from several both in terms of the OP or the instance of the commenter).
My gut says potentially issues with timezone somewhere and my offset (UTC+9) is potentially far enough out that it's an issue. I have no evidence for that. Looking at the request and response in dev tools hasn't yielded anything particularly useful so far as I can tell.