Ask: The HN dev(s): Upvotes are not counted - why?

I have recently noticed that when I want to upvote a story, my vote is not accepted/counted:

The upvote arrow disappears (as usual), but the vote counter stays exactly the same when the page is refreshed.

This happens both when I try to upvote something on the comments page and on the front page (list).

So, is this a bug or a feature?

If this is a feature, I it would be helpful to understand the reasoning behind it.

Thanks.