album: https://imgur.com/a/QcHG9VK
In the first screenshot, notice that the highlighted comment has no replies. Here's a link to the page I was on, anchored to that comment: https://news.ycombinator.com/item?p=3&id=27424195#27429786
In the second screenshot, after navigating to the comment directly, there are in fact replies: https://news.ycombinator.com/item?id=27429786
The bug is that those replies should be showing up in the first link, shouldn't they? E.g. https://news.ycombinator.com/item?id=27430242 is a reply to https://news.ycombinator.com/item?id=27429786, but when viewed on the original page https://news.ycombinator.com/item?p=3&id=27424195#27429786, it's absent.
I know that "more" pages shift toplevel comments around, but I've never seen child subtrees get completely pruned with no indication that they exist at all unless you navigate to the comment directly.
Sorry for the distraction. I didn't want to email this for a few reasons, some of which might be invalid. However, it does seem to be a slightly-serious bug if true, since a recent refactoring must've caused deeply-nested children not to show up when browsing top-level "more" links. I'm not sure anyone else has noticed yet.
(If this is intended behavior, sorry!)
Good luck bug hunting.