I hate a lot of the things that reddit has been doing but I still think not all of them are on purpose. I ran into this issue myself before it was widely discussed and my first thought was that it had simply failed to delete some comments or deleted only from some cache.
So far every exampe I’ve seen of this can still be explained by bad engineering and I see no reason to think it is “undeleting” stuff by design, since it seems to happen to very random content that has no general value (like restoring 20 random comments out of 900 that were deleted).