>However, clicking on the "past" link for that three year old submission does turn up the present submission.
The "past" feature is implemented by a third-party site, not HN.
Also, you can see the URL of the page and highlighted text on the page that it's searching by the title text as well as the link. The previous submission's title did not have all the words of the current one, hence does not show in the search results.
The current 'past' link isn't searching for the url at all. I think if it looked at the url instead of the title then it would miss when there are articles from other sites on the same topic so there is a bit of a tradeoff. If it looked at both, Algolia treats it as an 'and' so it would also not return the older submission. An 'or' would probably work well for this use case but not sure if it is supported.
This seems to have exposed a bug in HN. Clicking on the "past" link for the present submission does not turn up that past submission, although the links are identical.
However, clicking on the "past" link for that three year old submission does turn up the present submission.
[1] https://news.ycombinator.com/item?id=16635440