Actually it is shown but under an unrelated top-level comment by jbverschoor

Something got borked here.

Are you using any third-party extensions that could affect this? I can't see how our software would be producing pages that way. There's only one comment by that user (https://news.ycombinator.com/item?id=22757599) and it has no replies.

Nothing HN-specific, and yeah I also see jbverschoor as having only one comment without replies in his comments page and yet our entire conversation is shown in reply to this one comment on https://news.ycombinator.com/item?id=22756730

EDIT: scratch that, it shows in reply to hc91 [flagged] comment, but it in turn was originally a reply to my own, not a top-level comment for the submission.

Could it be just a showdead thing? The toplevel comment is flagged dead.

Good suggestion, but the software doesn't hide child comments of dead comments, even for users who don't have showdead.