My guess is that it's a bug caused by the new troll feeding penalty where you automatically lose 5 karma if you reply to a comment at -3 or below. But I don't really have any evidence to corroborate that.

And I don't think downvoted comments have ever been hidden if on a users page. I'm not sure though, I set my preferences to see all comments as soon as I created my LW account. I have negative associations with things being hidden from me and I'm too curious not to open it up and look anyway.