Question about something strange going on with the release of parts of a series of books.

Hello, I have a question which I hope someone can answer for me. I've been combing through several book series for updates and I came across something puzzling. For the book series "Heroine? Saint? No, I'm an All-Works Maid (and Proud of It)!

I noticed that the sixth volume on Audible has been released. Here's the thing, the fifth book is not available. Like the title is there but it's in the format of the books that seem to be planned to be released but haven't yet. You know, the one where it lists its release date to be "2200-01-01" obviously a placeholder date before putting down an official one.
So, how can the sixth volume be out when the fifth one has not been released yet?