Hi there.
New to this forum and had the same problem; guess that's actually no bug.
Previously converted books from AZW3 to EPUB perfectly but the latest one got me over 3000 pages instead of ~700
Actually i'm not exercised in html but comparing the html from a book that worked fine with the wrong one it occurs that the AZW3 download from Amazon got the command <p class="chapter"> a lot of times between the text...therefore it makes a 'pagebreak' on the reader...either in the Calibre reader and also in the physical E-Book Reader.
However, thx cybmole the solution from your primary post worked fine for me...