Workaround: Place the following script below the ScriptManager.

var $T = Telerik.Web.UI;
var $ = $ || $telerik.$
var _toggleActiveDescendantAttributes_old = $T.RadDropDownList._toggleActiveDescendantAttributes;

$T.RadDropDownList._toggleActiveDescendantAttributes = function (element, id, value) {
    if ($("#" + id + "_selected_item").length == 1) {
        $("#" + id + "_selected_item").removeAttr('id')
    }
    _toggleActiveDescendantAttributes_old.call(this, element, id, value);
}