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>