Tldr: If “Show read posts” is off, it hides seen posts. But it also hides them when viewing a communitys top posts and even on your own profile under “Posts”.

Let me explain: Lets say you click through your feed, see a bunch of posts from a community. Then you make a own post to that community.

After that you see that your post have blown up, so you want to see, if it is the topmost post in this community. So you search for the community and sort for “top all time”.

Now the problem: It in the settings you have “hide seen posts” you cannot see the posts, any posts, that you have seen previously, not even your own. And not even in your own profile, where every post is already read, so its eternally empty.

I think that being on a community should overrule the “hide seen” setting.