I agree that this is a failure mode, but it's not an absolute one: people could explain to you via PM. Then you'd be free to edit the comment, and if its score floated back up discussion could ensue below.
PMd explanations are not publicly visible, so they don't help others who read the thread and make the same mistake as the downvoted poster. They can't be upvoted by others, which removes a big (in my experience) incentive to post the explanations.
Has anyone yet mentioned or reported that for the last couple days, the "karma for last 30 days" is showing zero for everyone? And that we no longer can see the top contributors for the last 30 days either?
Do we have an explanation or estimation for a bugfix on this?