Ok you know what, I am really leaning on your getting messed with and someone is on your account.
Because a few things make no sense.
- the other series that got hit were one time, and the new chapters were left alone, this happened to around 50+ I read and this one alone loses new chapters.
- the deletion is handled differently, they didn't code a separate system just for that mass deletion, they used existing, there is not reason for the difference of it not showing the no longer exits list.
- a admin would not just manual delete chapters multiple times like this, they would give you a warning to stop uploading or ban.
Over all there is a reason a number of commenters suspected you were deleting them, what was happening just didn't make sense. And sounds like you confirmed your account was indeed the one that did. So yeah you really need to make sure your account is secure. And do so quickly, because now that you have posted awareness of it the person who is doing it may rush to do more damage.