OK. found the solution.
apparently, the problem appear when the main document height is shorter than the iframe
that is scrolled.
the parts of the iframe page, that exceed the document height, are not rendered.
So, under my needs, I could solve the problem by adding such a js (with jquery) code:
<script>
$(function() {
var iframe = $("#myIframe");
iframe.load(function() {
$("body").height(iframe.height());
});
});
</script>