I think this is because the "listview" plugin in jquery.mobile.js is too liberal in its parsing of list item contents.
It looks for ANY image, anywhere inside the li and then decides the li must be a thumbnail li.
I propose it makes much more sense to only use DIRECT DESCENDENTS of the li tag for figuring out whether the item has a header, subtext, thumbnail etc.
Here is what I modified to make this happen (but I think it would make sense for this to be in the mobile code, not just as a one-off hack)
lines 2531 ff
item.find( "li > h1, h2, h3, h4, h5, h6" ).addClass( "ui-li-heading" );
item.find( "li > p, dl" ).addClass( "ui-li-desc" );
item.find( "li > img" ).addClass( "ui-li-thumb" ).each(function() {
$( this ).closest( "li" )
.addClass( $(this).is( ".ui-li-icon" ) ? "ui-li-has-icon" : "ui-li-has-thumb" );
});
Hope this helps solve your problem.