Workaround:

<script>
    var $ = $telerik.$;
    Telerik.Web.UI.RadMenuItem.prototype._doAriaFocus = function () {
        var menu = this.get_menu();
        var menuId = menu.get_id() + "_active";

        $(this.get_element()).attr("id", menuId);
        $(menu._getMainElement()).attr("aria-activedescendant", menuId);
    }
</script>