Kind of falls under the 'Too Afraid to Ask' category, I guess, but I've been curious about this for a while. Did something actually happen at some point, or was this just a procedural thing that wasn't ever followed up on?
It's mildly annoying given how large they are.
Edit: It's possible that this isn't a federation problem at all (as discussion is bringing to light) but something else entirely. Regardless, though, something is going on.
It's also possible that the site I link below is out of date, so maybe don't take that as gospel. I bookmarked it a year ago and just hit it up to check on this a few minutes before posting, so I haven't been keeping up with it.
Doing a little more digging in light of the above, it's possible this is related to this issue, and there's just an extremely long delay before we get content from lemmy.world. Weirdly, though, it doesn't seem to be the case with other instances - maybe because of their size? Either way, looking at the same posts on our instance and 3 or 4 others, we seem to be the only ones not getting the replies. So something's fucked, maybe.
If you're on lemmy.world and happen to see this, drop a reply in here, maybe - I'd be curious to see how long it takes for us to see it (or if we can at all).
Took 2+ hours for this reply to show up to us, for the record.