According to the Mobipocket documentation:
The vertical space can also be forced by adding the height of a paragraph:
Code:
<p height="Y"> (pixels), <p height="Y%">,<p height="Yem"> or <p height="Ypt">
Y is an INTEGER and can be only positive.
So, it seems "0.5em" is not valid and maybe mobipocket creator converts it to "1em" or something similar...