Workaround: using the following handler for the OnClientLoad event of the RibbonBar
<script>
function OnClientLoad(sender, args) {
var oldHide = sender._hideMenus;
sender._hideMenus = function () {
var ribbonBar = this,
visibleMenus = $telerik.$(sender.get_element()).find("." + ribbonBar._cssClasses._rrbMenuGroupOut).filter(":visible").get();
for (i = 0, len = visibleMenus.length; i < len; i++) {
var menu = visibleMenus[i];
menu._button.removeClass("rrbExpanded");
}
oldHide.call(this)
}
}
</script>