C# - GridView - Columns.Insert() Cause Data To Vanish On Postback But Columns.Add() Works OK?

I have a GridView that has several dynamic columns (I do not know how many at design time and it could be 0-12 columns, hence need for dynamic columns). I have the columns in the grid and data bound to them - works great. There are other standard, design-time TemplateField columns with TextBox controls in them. These are bound with values that the user can edit. The grid is posted back via a Submit button. My question is "Why does gv.Columns.Insert() cause all my TextBox data to be null on Postback, but gv.Columns.Add() works like a champ?"

protected void BuildColumns()
{
// The first column to begin to insert the columns in the GridView
int columnIndex = 5;
BoundField aoColumn = new BoundField();
aoColumn.HeaderText = "New Column 1";
gvMyGrid.Columns.Insert(columnIndex, aoColumn); // kills txtQuantity.Text on postback
gvMyGrid.Columns.Add(aoColumn); // works fine
columnIndex++;
foreach (MyEntity my in _myEntityCollection)
{
BoundField myColumn = new BoundField();
myColumn.HeaderText = String.Format("{0:d}", my.StartDate);
gvMyGrid.Columns.Insert(columnIndex, myColumn);
columnIndex++;
}
}

I then go on to assign values to these BoundFields in the _RowDataBound method and all of this works great. However, when I post back and try to reference some TextBox and they are all null. And yes, I have the BuildColumns() call wrapped in if (!IsPostBack) on Page_Load. Of course I would like to use .Insert() so that the columns can go in the proper location and not at the end of the Columns array.