Downvote arrows disappear after the time window for downvoting is up, and that time window is shorter than for upvoting. Also, no one can downvote direct replies to themselves.

It's better to send questions like this to hn@ycombinator.com, though, as the site guidelines (https://news.ycombinator.com/newsguidelines.html) ask.

We detached this comment from https://news.ycombinator.com/item?id=12023677 and marked it off-topic.