What I can suggest is setting the foreignKeyRootValue option of igTreeGrid. option specifies the foreign key value in the data source to treat as the root level once the grid is data bound. Its default value is -1 (which includes the entire data source). If the foreign key field is of a typr string  the foreignKeyRootValue should be set to the exact same string. For example:

 var flatDS = [
        { "employeeID": "0", "PID": "-1", "firstName": "Andrew", "lastName": "Fuller", "reportsTo": "-" },
        { "employeeID": "2", "PID": "-1", "firstName": "Andrew", "lastName": "Muller", "reportsTo": "-" },
 
        { "employeeID": "4", "PID": "0", "firstName": "Janet", "lastName": "Leverling", "reportsTo": "0" },
        { "employeeID": "5", "PID": "0", "firstName": "Laura", "lastName": "Callahan", "reportsTo": "0" },
        { "employeeID": "6", "PID": "0", "firstName": "Margaret", "lastName": "Peacock", "reportsTo": "0" },
        { "employeeID": "7", "PID": "0", "firstName": "Michael", "lastName": "Suyama", "reportsTo": "0" },
        // sub of ID 4
        { "employeeID": "8", "PID": "4", "firstName": "Anne", "lastName": "Dodsworth", "reportsTo": "4" },
        { "employeeID": "9", "PID": "4", "firstName": "Danielle", "lastName": "Davis", "reportsTo": "4" },
        { "employeeID": "10", "PID": "4", "firstName": "Robert", "lastName": "King", "reportsTo": "4" },
        // sub of ID 2
        { "employeeID": "11", "PID": "2", "firstName": "Peter", "lastName": "Lewis", "reportsTo": "2" },
        { "employeeID": "12", "PID": "2", "firstName": "Ryder", "lastName": "Zenaida", "reportsTo": "2" },
        { "employeeID": "13", "PID": "2", "firstName": "Wang", "lastName": "Mercedes", "reportsTo": "2" }
    ];
    $("#treegrid1").igTreeGrid({
        width: "100%",
        dataSource: flatDS, //bound to flat data source,
        autoGenerateColumns: false,
        primaryKey: "employeeID",
        foreignKey: "PID",
        initialExpandDepth: 1,
        foreignKeyRootValue: "-1",
        columns: [
            { headerText: "Employee ID", key: "employeeID", width: "200px", dataType: "string" },
            { headerText: "First Name", key: "firstName", width: "220px", dataType: "string" },
            { headerText: "Last Name", key: "lastName", width: "220px", dataType: "string" },
            { headerText: "Reports To", key: "reportsTo", width: "130px", dataType: "number" }
        ]
    });

I am attaching a small sample for your reference. Please test it on your side and let me know if you have any additional questions regarding this matter.

Oops ... mistakenly posted to igTree rather than igTreeGrid.  Moved the question over there.