Strangely, it seems the HN search will exact match on the initial search and then prefix match after that. Compare URLs:
Add an 's' then remove it
I've submitted an issue on GitHub to address this: https://github.com/algolia/hn-search/issues/41
But I found that I rely on intra line alignment often and proportional fonts cannot deliver that. The end result is so ugly as to completely overwhelm the advantages.
I ended up returning to M+.