I think this is actually an Algolia search problem. It appears to be missing recent items too. hnrss.org uses the Algolia REST API

I deleted and re-added the feed when I noticed it hadn't updated for some time and I have more recent articles. Not sure if something like the URL changed? You can see more recent posts directly

https://news.ycombinator.com/rss

No updates since 3:11pm PDT yesterday.

Same here.