Darth_Xiane
Which book is it? Is it original or translated?
In the case of translated books, as far as I know, translation groups just have too many books on their plates. And it could sometimes happen that the coordinator of a group fails to keep track of which books have been completed and which haven't, and therefore forgets to set the status of those books to completed.
The scenario above seemed highly unlikely In the case of an original book because I don't think there were any original authors on this site that have too many books to handle.
Also, most translation group coordinators don't read the comments section of their books. So, if the book in your case is a translated one, it could indeed be an oversight. If you know the translation group, you can head over to their discord server and ping them there. But if you don't know, you can also just tag CKtalon here or over on the webnovel's discord server (head over to the #suggestions_bug_feedback channel), and he'll take care of it.
In the case of an original book, I'm not sure what you could do. Maybe report the author to Webnovel?