Just upvoted your comment, refresh page and the upvote button reappears :-|
and I just upvoted a comment, refreshed, and it's still upvoted - perhaps check your network stack in your browser? Sounds like something isn't getting through and the UI is unaware.
I had js disabled on HN (and it's been that way forever), enabling it restores upvote functionality. Must have been some js-ification of that part of the site's code; problem solved, many thanks.