Mutli Level Group Bug

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.14/themes/base/jquery-ui.css">
<script src="https://dl.dropbox.com/u/16478914/grid.locale-en.js"></script>
<link rel="stylesheet" href="https://dl.dropbox.com/u/16478914/ui.jqgrid.css">
<script src="https://dl.dropbox.com/u/16478914/jquery.jqGrid.src.js"></script>
<table id="grid"></table>

JavaScript

var data = [
    {
    "ExtensionData": {},
    "Brand": "",
    "Category": "Desserts & Mixes",
    "CategoryNumber": "2016",
    "Department": "BAKERY",
    "DepartmentNumber": "944",
    "id": 1,
    "PriceAssociationCode": 2,
    "RetailGroup": "Breads",
    "SellSize": "16 OZ",
    "ItemID": 0,
    "ListID": 0,
    "CategoryID": 0,
    "UPC": 240076000003,
    "Description": "Southern Style Corn Bread",
    "Quantity": 0,
    "Priority": 0,
    "IsDeleted": false,
    "ItemCategory": null},
{
    "ExtensionData": {},
    "Brand": "Fresh Baked",
    "Category": "Breads",
    "CategoryNumber": "2008",
    "Department": "BAKERY",
    "DepartmentNumber": "944",
    "id": 2,
    "PriceAssociationCode": 2,
    "RetailGroup": "Breads",
    "SellSize": "14.5 OZ",
    "ItemID": 0,
    "ListID": 0,
    "CategoryID": 0,
    "UPC": 781421002339,
    "Description": "Rosemary Bread",
    "Quantity": 0,
    "Priority": 0,
    "IsDeleted": false,
    "ItemCategory": null},
{
    "ExtensionData": {},
    "Brand": "In Our Bakery",
    "Category": "Desserts & Mixes",
    "CategoryNumber": "2016",
    "Department": "BAKERY",
    "DepartmentNumber": "944",
    "id": 3,
    "PriceAssociationCode": 600200,
    "RetailGroup": "NUT BREADS",
    "SellSize": "16 OZ",
    "ItemID": 0,
    "ListID": 0,
    "CategoryID": 0,
    "UPC": 240359000003,
    "Description": "Pumpkin Nut Bread",
    "Quantity": 0,
    "Priority": 0,
    "IsDeleted": false,
    "ItemCategory": null},
{
    "ExtensionData": {},
    "Brand": "In Our Bakery",
    "Category": "Desserts & Mixes",
    "CategoryNumber": "2016",
    "Department": "BAKERY",
    "DepartmentNumber": "944",
    "id": 4,
    "PriceAssociationCode": 600200,
    "RetailGroup": "NUT BREADS",
    "SellSize": "16 OZ",
    "ItemID": 0,
    "ListID": 0,
    "CategoryID": 0,
    "UPC": 241455000003,
    "Description": "Cranberry Nut Bread",
    "Quantity": 0,
    "Priority": 0,
    "IsDeleted": false,
    "ItemCategory":...