Have you checked toc.ncx and toc.xhtml have the same order?
I've seen a few cases when they had a different order and it caused problems with some readers which use toc.ncx as a guide to order the epub pages.
You can check both using any text editor.

@Matreyu

Hello,

Unfortunately, there is no hard date for when pre-publication will start for V8 of Potions.

For a general overview of how new volume starts are handled at J-Novel Club, please refer to the following topic: https://forums.j-novel.club/topic/3478/future-volume-release-schedule

Thank you