It is already well-known behavior which is being documented.
Unfortunatelly it's just a problem on the way lemmy works at the moment. I'm sure it'll get better with future upgrades.

A lot of it currently is just down to issues with federation. Like another user posted, you can trigger a sync of new posts and comments manually, so that should help in cases where you're after a specific post or comment.

A more long term solution will be optimizing Lemmy federation code and improving Lemmy instance server resources for bigger instances. Both are actively being worked on!

Yeah I've started noticing that a lot today. Even had a "ghost reply" to a comment that wasn't showing up here at all, but appeared in my inbox. Looking at things at their original instance, both my comment and the reply showed up fine, just not here.

I tried https://join-lemmy.org/... and now https://lemm.ee/post/26360 and https://feddit.it/post/300204 seem to be synced - both have 10 comments.

I’ve been seeing this quite a lot. Not came across a definitive answer but from what I can gather it’s an issue with instances federating comments. I wonder if it’s an issue just now due to the massive growth and the stress on instances. @sunaurus@lemm.ee may be able to offer a better insight.

Man, this takes me back to when I hooked my BBS up to fidomail in the late 80’s early 90’s. Same growing pains but the hackers (in the original sense of the word) will prevail.