The reason why it's broken is because the query to update the rankings were so slow that it only ran once a week or so. So unless you restarted your instance every day the ranking would be stale.
Fortunately there's a fix available in the next Lemmy version, 0.18.3. If you are impatient you could build an image of the release candidate (git tag is 0.18.3-rc.2
) and deploy it on your server.