There's something I don't understand: YenPress did publish this once and it's still buyable, despite the fact that they have removed almost all of the related content from their website (only a broken link on their utterly incomplete "series" page remains). The thing is that YP published 7 volumes (last one in 2015, but 5th one in 2011), and the cover of the 7th is exactly the one shown above, which seemingly belongs to the 9th.
Does someone know if YP just rearranged the chapters in less volumes (and in that case, which published volume matches the last translated chapter)?