Based on what I have read, the root cause is probably that Reddit doesn’t actually delete comments, it just marks them in the database as “this is not supposed to be displayed” or something like that. That means that it’s pretty easy for “deleted” comments to reappear without anyone actually trying to revert them, because the “deleted” flag gets lost somehow. That can happen in any number of ways, and it’s unlikely that it’s admins deliberately undoing user deletions, but it is happening. I have been manually deleting, so it isn’t just because scripts don’t get all the comments the first time (though that seems to be a common problem), and I can confirm that some comments in private subs couldn’t be deleted until after the subs opened up again, but that some things also popped back up with no changes to the subs in question. Yes, a lot of comments are “reverting” because they weren’t deleted in the first place, but that definitely doesn’t cover all of the issues people are having.

I am not positive that “delete” flags rather than actual deletion is what’s happening technologically, but it would explain how often deleted material is popping back up for people. Properly deleted comments shouldn’t show back up even if subreddits are toggled from private to public and back, because deleted comments should be gone as soon as you are able to delete them in the first place. This kind of fake deletion is apparently not unusual for sites that store and display content, but it doesn’t comply with EU privacy laws, precisely because it’s so easy for “deleted” material to become public again.

And that’s really the point: it doesn’t matter exactly why specific content is popping back up. The fact that any content is popping back up after a user tries to delete it means something is wrong, and if enough EU users complain, there will be grounds for a lawsuit against Reddit, regardless of whether anyone at Reddit is deliberately doing anything wrong.