This is not a glitch. AskHistorians is an actively-moderated subreddit. Answers which do not meet our standards are removed, as are comments that violate our guideline (links to wikipedia, suggestions that the questioner google the answer, suggested sex acts for moderators to perform on themselves, "where have all the comments gone?" etcetera, etcetera.) Very popular questions can attract a lot of likes, and a lot of low-quality comments, in a short period of time - but questions often take hours, sometimes days to arrive at a good answer. So you have the "comment graveyard" effect where everything is removed on a very popular question.

Unfortunately, due to the nature of reddit, removed comments still show up in the totals, unless you use our Browser Extension.