I've noticed something similar with AI assist authored books as well. Early on it does alright, but after some chapters the beginning of each chapter repeats the end of the previous, and obvious LLM tells become more frequent.

The more it has to go on, the more it relies on repetition of what came before. It's also possible that authors start paying much less attention and put less effort into editing later chapters.

Despite the sheer volume on Amazon, LLMs are not at the point of writing well.