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 can see them on Sync for Lemmy.
Yes, because you are on lemm.ee as well. See the same post in browser from different instances, e.g.: you have a comment on this post: https://lemm.ee/post/19222111 If you see the same post on lemmy.world, you cannot find your comment: https://lemmy.world/post/10009901