So, to explain in more detail: I read about a new community on newcommunities and I decided to search for it so that it started federating with Kbin.
However, when I visit the community on lemmy.world, there are already 32 posts from before the community became visible on kbin, but I can only see the posts that were made after I “discovered” the community on Kbin.
Interesting, it seems to work the same way between Lemmy instances. It would be nice if I could force a more thorough federation, but it’s good to see most content will eventually come through on its own time.