jQuery Grid Plugin - jqGrid

Hello im using jqGrid 4.4.1

with datatype:"local".

first:

i have the grid filled with 2 records/rows when the grid init for the first time (Rows ID=10 and ID=20).

then: i call an ajax to fill the grid with more records (about 900 rows), im calling addJSONData method, the grid show the 900 rows BUT... when i filter by last name, name or any input (with filtertoolbar) and hit ENTER, the grid goes blank, then when i click to clean/reset/restore the filters, the grid fill up/charge ONLY the 2 first rows of the first time (Rows ID=10 and ID=20, the new 900 rows dont seem to charge).

Its seems that the grid is applying the filter internally ONLY with the 2 first values of the init time (Rows ID=10 and ID=20).

before using addJSONData i use jqGrid('addRowData') BUT with more volume of data the performance crash, thats why i must use addJSONData.

Thanks.

Code:

    var mydata = [        
            {
            id:"10",firstname:"SAMPLE1",lastname:"SAMPLE10"
            },
            {
            id:"20",firstname:"SAMPLE2",lastname:"SAMPLE20"
            }
        ];
        
        jQuery("#myGrid").jqGrid(
            {
                data: mydata,
                datatype: "local",
                colNames:['ID','First Name', 'Last Name'],
                colModel:[
                        {name:'id',index:'id', width:250, sortable:false, editable:false, sorttype:'text',search:true,searchoptions:{sopt:['cn','nc']},searchhidden:true},
                        {name:'firstname',index:'firstname', width:250, sortable:false, editable:false, sorttype:'text',search:true,searchoptions:{sopt:['cn','nc']},searchhidden:true},
                        {name:'lastname',index:'lastname', width:250, sortable:false, editable:false, sorttype:'text',search:true,searchoptions:{sopt:['cn','nc']},searchhidden:true}
                ],
                rowNum:1000,
                rowList:[10,20,30,50,100,1000],
                height: 350,
                width: 1000,
                rowTotal:500000,
                pager: '#pmyGrid',
                sortname: 'firstname',
                sortorder: "asc",
                viewrecords: true,
                caption:"Grid",
                //shrinkToFit:false,
                footerrow: true,
                userDataOnFooter: true,    
            }
        );
        jQuery("#myGrid").jqGrid('navGrid','#pmyGrid',{add:false,edit:false,del:false});
        jQuery("#myGrid").jqGrid('bindKeys');
        jQuery("#myGrid").jqGrid('filterToolbar', {
                   beforeSearch: function() {
                   }
                 }
        );

To fill up i use:

        eval("mydata="+partetxt+";");
        jQuery("#myGrid")[0].addJSONData(mydata); //Fill well but dont see in new filters.