There is currently a bug in Lemmy 0.17.4 that causes post rank calculation to stop shortly after the server is restarted. It seems like lemmy.world is affected as well.
You can track this bug here: https://github.com/LemmyNet/lemmy/issues/3076
I pinged Ruud about this, he can restart the server for now for the ranks to refresh, but I am also working on investigating and fixing the root cause.