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.

I think there might be more going on as the post in question is 4 days old, and all the comments are at least 1 day old; is the syncing typically days behind? The federated version that OP links shows 0 comments, and the version on my instance has just 5 comments.

Is it to do with when a user on the remote instance first interacted with the post? I.e, its only showing comments from after someone on lemmy.sdf.org first interacted with the post?

Comments from before the first user subscribes simply do not sync. An instance only gets comments made after at least one user subscribes to a community.

Would you mind elaborating on this a bit or linking to documentation that explains this in more detail? It seems that unless I visit any thread on its own instance, I'm not getting the actual number of upvotes and/or comments. Waiting and checking back doesn't seem to solve it (double checked on a post I checked about 4 hours ago, still can't see the full thing), so maybe it helps if I know what's going on exactly.

your replay came quickly… so syncing seems to be working. If something doesn’t get synced is it retried again later? A good chunk of comments are missing across many posts. 😔

In my example the post is ~4 days old and the comments are as recent as a day ago. I can see the post on my instance but zero comments.

Are all posts synced but not comments if I’m the first to subscribe to a community on my instance?

I have "Undetermined" and "English" selected. There is no change to the comment count if I keep adding languages.

But, that should only matter when viewing via a logged in server. Those two links are to communities directly on servers. I suppose it’s possible no one on lemmy.sdf.org has subscribed to that community. That could explain the original question but doesn’t explain the example posted by @caio@sh.itjust.works example of https://lemmy.ml/post/1136642 vs https://lemmy.fdvrs.xyz/post/1436

(How do I get a link to reference a comment that works everywhere?)

If you are the first to subscribe to one from another region, it takes a while for comments to sync. Once it does, you can read and comment to your heart's content.

Well, you CAN participate, subscribe and read the community of another instance. You don't have to be part of the other instance to do so.

Just make sure you are browsing / searching all and not just local.

I am reading and writing this from kbin and not even Lemmy btw.

That doesn't really make sense though - shouldn't comments be federated across all communities?