MWEs don't get much more minimal than this:
\documentclass[]{book}
\begin{document}
This is \emph{very} strange
\end{document}
You can compile it using tex4ebook, but htlatex example.tex xhtml
has the same result: if you run the epub or .html output through the Kindle Previewer, it displays just fine on "Kindle Paperwhite," but on "Kindle DX" the italicized "very" appears indented on its very own line. "This is" and "strange" are both flushed left.
(I actually had a similar problem on the "Paperwhite" previewer when I was trying to use the special "dictionary" <idx>
tags, and when I downloaded the file to an actual device it had the same problem, so I don't think this is just a Previewer bug, but I don't have a Kindle DX...)
I realize that this may be beyond the scope of tex.stackexchange, but I am open to recommendations of other places to post. I suspect that it may be possible to fix it by changing the tex4ht-generated css, though.
Thanks!