For example, I’m on Lemmy.sdf.org and I joined the Apple@lemmy.ml community. But there are many missing articles and comments. If I browse directly on Lemmy.ml for the same article there ~90% of comments are missing.
https://lemmy.ml/post/1152794 has 26 comments https://lemmy.sdf.org/post/11232 has 0 comments
Pretty sure it doesn’t “take a while” for things to sync up. Once an instance is discovered by another, any posts from that point forward will appear, but anything made before then will not. I have personally seen new instances sync with kbin.social that have less comments than if you look at the main instance’s thread. They have added more comments, but only new ones made after it was discovered.
Whether I’m right or not, I’m not sure. Would have to hear from the devs to know for sure, but from all I’ve seen there is no syncing of archived or historical postings, just discovery of and acceptance of new data from other instances.
This is my understanding as well, after using a number of different instances over the last few months.
I believe this issue will mostly sort itself out in the future as the federated network becomes more “connected”
A new instance connecting to an existing one will have content from the established instance missing, but as time goes on that content becomes a smaller portion of the overall content.
I anticipate some development on this issue in the near future anyway, perhaps cached content that is able to be “pushed” to a requesting instance.