...

 on a client, we would access that property like this:

wgGetCellByName(SelectedRow,"EmployeeId").EmployeeName;

and that worked in IE8 and below, however it doesn't work in HTML5 browsers. With the new DOCTYPE html, in new browsers, that gets rendered as a direct property on a combo object, while in older browsers it's rendered as a member of attributes array on a combo object in js.

...

I recommend you to switch your JS code to the new client-side API of WebGrid. In order to get value from hidden column using HiddenDataMember, you can use following snippet code:

function GetValueFromHiddenColumnUsingHiddenDataMember()
{
    // get the client object reference of WebGrid1
    var grid = ISGetObject("WebGrid1");

    // get the row object of the selected object
    var rowObj = grid.GetSelectedObject().GetRowObject();

    // get cells of the row object
    var cells = rowObj.GetCells();

    // get EmployeeId cell
    var employee = cells.GetNamedItem("EmployeeId");


    var employeeName = customer.GetElement().attributes["EmployeeName"].value;

    alert(employeeName);
}

Have you encountered this issue and is there some solutio to it?

I can reproduce the problem when using wgGetCellByName. The problem no longer persist when the recent client-side API is used.

This should helps.