I couldn't find that. I did find a setting called "Show read posts", but it was already unchecked...

I'm guessing it's because lemmy afaik doesn't actually have any support for post hiding, so Sync is just putting it off to one side out of view, but then that's not saved for when you reload the app. Hopefully we can get proper post hiding (or at least a workaround) soon, I browse the same way you do