Take https://lemm.ee/c/moviesandtv for example. From lemmy.ml, lemmy.world, and sh.itjust.works, you can’t see any posts or comments from lemm.ee users, and lemm.ee users can’t see any posts from other instances.
I don’t know what’s broken, but something is badly broken.
I believe that an issue relating to federation was found with version 0.19, and has been addressed in 0.19.1, so we have to wait for the admins to upgrade again.