Clicking where they should be, I see comments get the "unvote" action, so it seems to be a matter of visibility.
When I first noticed the buttons were missing, I was reading the Hans Niemann cheating report thread and thought voting might have been intentionally disabled for that thread. Seeing the same thing in other threads though, so alas this does not appear to be an intentional move to prevent chess flame wars.
Try sending a screenshot to the mods hn@ycombinator.com with all the info about your browser and os.
I usually see missing vote buttons when I'm browsing without images.