update callback is not called everytime

I'm using nested ul/li lists with sortable and when I drag the first li-element from one ul-list to the top of another ul-list, the update callback is not called.

Example:

<ul id="mylist">
	<li>foo</li>
	<li>bar</li>
	<li>
		zyx
		<ul>
			<li>zyx-1</li>
			<li>zyx-2</li>
		</ul>
	</li>
</ul>
<script type="text/javascript">
$('#mylist').sortable({items: 'li', update: function(e,ui) {
	alert('update');
}});
</script>

Update is not called if I drag the first li-element from the top level ul-list to be the first element of the second ul-list.

I also tried using the connectWith option. There the update callback is not called if you drag the first li-element to be the first element of another ul-list under the parent ul in dom-tree.

Also update is called twice when dragging any li-element to ul-list above the parent ul.

Tested with 1.5 and with 26.6. version from subversion.