Edit: After reading your comments, and testing some more, I must say that I've misunderstood how it all works.
I should've thought of Mastodon users like separate Lemmy communities...but not exactly. What confused me is the fact that you could look up a profile on a remote instance and see their posts, but they would be very delayed. On Lemmy, if your instance hasn't "discovered" a community, you wouldn't see it at all.