That's very strange. I'm thinking that rather than it being Apple's fault, it's the way that one of those audiobooks has been encoded - it's telling your phone that it's music rather than an audio book. I'm sure years ago I read something about how to change the file identifier for that purpose if you were on a Mac - but since you are using it on an iPhone, I'm not so sure. Just out of interest, which audio book is misbehaving? I might take a look.
Galaxy Note 8 vs iPhone X