I'd return this book... your characters are all messed up with strange squiggles through them, and all the letters are in the wrong order. I can't make heads or tails of it
Does the book still hyphenate correctly when you change the font-size, or is this not possible? It could be that the book has a built-in font, set at a specific size, and that the hyphenation is hardcoded in the book.
You could try, for example, to remove the font from the book (convert to EPUB and use the Modify EPUB plugin) plugin and remove any "font-size" attributes in the CSS (use Sigil) for anything but the chapters. Then reconvert to KF8 and re-upload the book onto the Kindle so you can change fonts and sizes.
I wouldn't be surprised if you start seeing very weird hyphenation in places where there shouldn't be any.