See screenshot. Example here is !privacyguides@lemmy.ml On lemmy.ml I can see several posts, on lemmy.world, none.
I logged in in both, I joined both, both profiles have enabled NSFW content and language is untouched (so all languages should be displayed).
What is the reason for that and how I get the posts on lemmy.world?
Correct. It does (sometimes) backfill a a page of old posts on first subscriber, but zero comments. There is no backfill procedure for new servers.
Right now Lemmy has pretty serious bugs, such as not sending comment deletes to all servers subscribed to a community. So things like backfill aren't getting as much attention because it's hard to even keep servers consistent with new content.