I can't tell if it's incompetence or malice, but this happened after the redesign. I think it's caused by the presence of recent posts from the "more posts" section.

Another consequence of the redesign, much worse than useless dates, is that you get completely useless results just because the same "related post" (the title of which Google happens to deem "relevant") appears over and over again at the bottom of different pages.

To get around this you can search for "site:old.reddit.com" or "site:i.reddit.com". One downside is you get fewer results.