From what I understand, all comments are hosted on their original server, but contain pointers that tell it what it’s parent comment is. These are then cashed to each viewing server. So if instance a goes down, to my knowledge someone viewing from instance b would still see it. But if instance C came along and never saw it before, it could never discover the comment. That’s at least my understanding, an actual Dev or someone more familiar with activity pub would give you a better answer.