I used "Power Delete Suite" to delete all my comments. Everything over 3 months old, came back a little over 24 hours later.

Edit: Now less sure of this statement. I just had more "come back", all in the same subreddit. So it now appears I'm seeing comments I couldn't previously see because that sub was still private. This makes me less sure the comments I saw previously "came back" rather than being the same issue.