Dang: "The problem is likely at Algolia's end. They also have a nice HN Search API and most likely hnrss.org is using that one. I've contacted them and they're currently looking into it.

Edit: they've found the problem and are currently backfilling the missing data."

https://news.ycombinator.com/item?id=33790811

Thanks!

Not everything is indexed. This isn't documented anywhere.