What may cause comments to lose all of their upvotes?
I frequently come across older posts because I get email notifications for - I'm guessing - a few thousand threads, and because I often search for older info to cross-reference when helping people.
Hence, I've noticed on a number of occasions that comments left either by myself or by others suddenly lose all of their upvotes. This is obviously most noticable when they had lots of them.
For instance, in https://forum.duolingo.com/comment/7095727, my reply to the top-level comment lost all of its upvotes a while ago. And yesterday, Lundgren8's Venn diagram reply - which had several hundred upvotes - lost all upvotes except, apparently, mine.
Another example can be found in https://forum.duolingo.com/comment/8618096 where my comment used to have lots more upvotes.
I don't really care about the upvotes on my comments (or I would have reported this much earlier), but I've recently started noticing this happen to posts by other people as well, and it does annoy me that it kills the established comment hierarchy.
This happened to some high profile language request posts a while back, including the main ones for Finnish and Latin, and to the announcement post when Activity was removed (though that had negative votes instead of upvotes). As far as I know no cause was ever identified, though some users jumped to the conclusion that it was mega vote spamming or Duo intentionally resetting the votes with malicious intent.
As anyone who thinks about it should hopefully know, since if Duolingo wanted to do that for any malicious purpose they're going about it in the worst, most illogical possible way. :) (Resetting them randomly on a few and leaving the rest, and randomly resetting occasional perfectly good posts with no plausible reason for doing so? That's the best tactic for censorship, especially when you want to hide it!)