Yeah, just recently saw a news article pop up in my Hot feed which stated that "Canada finally starts requiring a negative COVID-19 test to enter the country". Weirded me out for a moment before I realized that the post was over two years old :)
At least was a bug. If a post for whatever reason is requested again from another instance, then it would be inserted into the database with a very high hot score, as if it was a new post. But I thought that was fixed.
Its a known bug and 0.18 already improved it a bit. O.18.1 will hopefully fix it completely. For now the instance admin can restart the Lemmy backend to renew the stalled hot sorting.
The sorting algorithm seems like it needs a bit of work. I don't have the issue that I see month old posts, I tend to see very new and not very upvoted posts instead, but that's fine with me. I just saw your post on hot near the top page, for example.