This is a very common problem in this subreddit. For example, this thread says there's 18 comments as of this writing, but in reality it only contains 4 notices that a comment has been deleted and a mod's comment explaining why a comment has been deleted. There's no actual content whatsoever.
I understand this is probably a technical issue over which the mods have little control but maybe they could report it higher?
Unfortunately, the moderators of /r/AskHistorians don't have any control over how Reddit displays the amount of comments in a thread vs how many you actually see. We wish we did have control over that, as it annoys a lot of users who just want to see great history writing (including us!) and don't want to click on a thread that Reddit reckons has comments, only to discover a graveyard. Sadly, there's probably good reasons for Reddit doing it that way.
My understanding is that the way it works at present is in part a sort of anti-spam measure by the admins - because of Reddit's policy about this, spammers don't know whether their post has been removed or seen by people; this Schrodinger's spam situation discourages them. For what it's worth, in the thread linked to, the majority of the comments are in a now-removed thread where a) someone posted something that wasn't attempting to answer the question, b) someone wondered why they'd gotten so many downvotes, and then c) about ten different people tried to explain the various /r/AskHistorians rules the original comment had broken. We then removed the entire thread, because it's clutter.