This is an issue that the dev team is actively working on and (I believe) it'll be a part of the 0.18.0 release.
There's a hacky workaround some admins in the matrix chat implemented @TheDude@sh.itjust.works -- set a cron job to restart your lemmy back-end every hour or so. Not sure what the impact would be on a bigger instance like this but the restart only take a few seconds on my smaller one and it keeps my feeds fresh.