I had a somewhat analogous experience with Seveneves, which many people have recommended endlessly.

At some point in the book, around half way IIRC, there is an event that happens which explains the title. And it goes from something like an interesting hard sci-fi, to a sort of weird meandering fantasy thing that took a nuke to my suspension of disbelief. It also felt like the book was basically two entirely different books at that point. I simply could not get into it, or passed it.