Well, the free sample I downloaded was AZW3 (KF8) and it's a mess as far as the html/css is concerned. Lots of this:
<p class="JonnaMemoir" style="margin-bottom:10.0pt;text-align:justify;text-indent:
.3in;line-height:115%"><span style="font-family:"Times New Roman"">“You went
back to the office?” I was completely confused. If ever there were a doctor’s
note to get out of work, “brain tumor” had to be at the top of the list.</span></p><o:p></o:p>
I did a AZW3 to Mobi conversion, filtering "fonts" and it seemed to do the trick. Not sure what's happening on your end. Maybe the full book is different.
I ran in to the same problem here with a KF8 (azw3) book: