This is a 100% AI story. Opened a random chapter, and it's not even close (mixed/edited) but fully AI, confirmed by three different sites and my own eyes, which are already making my brain hurt.
It takes a while until you realize it, but even subconsciously, you get tired of the AI phrasing.
Maybe you didn't generate it, but you sure as well used it to fully rewrite/edit your story. Pretty sure that's the main reason for the drops. "Only" losing 8 out of around 40 is relatively low and pretty good for AI. I just need to glance at your choice of words when you write in this thread and then compare it with your story.
Anyway, no one here on SH appreciates the AI's attempt at "serious writing". It's not even about the plot but about the choice of words and general style. NOBODY talks like that! NOBODY narrates like that. Only AI does it that way.