This may be a complete red herring, but I'll pass it on anyway.
Have you looked at the XHTML code inside the EPUB to see if you can spot anything unusual? I have occasionally seen forum posts from other members who have had problems with words "running off the sides". When they checked the XHTML they found that some of the text was wrapped in <pre> tags rather than <p> tags.
Of course, as an English-only speaker, I have no experience with the challenges associated with using Chinese fonts.