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.
No.
Darn, that’s a bummer. :(Edit: It turns out that, as per czech’s answer you can let the posts federate by interacting with them on the remote.