Why is findcontrol in gridview not working after gridview.column.removeat

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C#

Gridview

I want to have a gridview control in C#.net that it can toggle between 15 and 10 columns depending on a boolean variable.
I have here the method wherein I will pass the DataTable and it renders it properly.

private void RenderDataTableToGrid(DataTable dt)
{
    //initialize controls
    int gvcolCount = gvMandayEntry.Columns.Count;
    pjkMain = itfinance_dataload_pjk.GetPjkMainRecord(PjkId);
    IsCarryFoward = IsProjectCarryForward(pjkMain);

    List<xreffy> FYlist = XrefTable(dt);
    for (int cols = 2; cols < dt.Columns.Count; cols++)
    {
        gvMandayEntry.Columns[cols].HeaderText = dt.Columns[cols].ColumnName;
    }

    if (!IsCarryFoward)
    {
        while (dt.Columns.Count != gvMandayEntry.Columns.Count)
        {
            gvMandayEntry.Columns.RemoveAt(12);
        }
    }
    //Load Data
    gvMandayEntry.DataSource = dt;
    gvMandayEntry.DataBind();

    for (int x = 0; x < dt.Rows.Count; x++)
    {
        Label Category = (Label)gvMandayEntry.Rows[x].Cells[0].FindControl("Categories");
        Label Type = (Label)gvMandayEntry.Rows[x].Cells[1].FindControl("Type");

        Category.Text = dt.Rows[x][0].ToString();
        Type.Text = dt.Rows[x][1].ToString();

        for (int y = 2; y < dt.Columns.Count; y++)
        {
            TextBox txt = (TextBox)gvMandayEntry.Rows[x].Cells[y].FindControl(FYlist.Find(fy => fy.finyear == dt.Columns[y].ColumnName).control_name);
            txt.Text = dt.Rows[x][y].ToString();
        }
    }
    divMandaysEntry.Visible = true;

    Session["dtManday"] = dt;
}

When its time for the user to click Save, the logic of the code is to readback the GridView per row using "FindControl".
It works fine if gvMandayEntry.Columns.RemoveAt(12); line from RenderDataTableToGrid was not executed at runtime.

But if its executed, the FindControl returns a Null.

for (int x = 0; x < gvMandayEntry.Rows.Count; x++)
{
    Label Category = (Label)gvMandayEntry.Rows[x].Cells[0].FindControl("Categories");
    Label Type = (Label)gvMandayEntry.Rows[x].Cells[1].FindControl("Type");

    int manday_category_id = MdCatList.Find(mdcat => mdcat.name == Category.Text).id;
    int pex_id = ExpenseType.Find(pex => pex.name == Type.Text).id;
    int manday_id = MdList.Find(mditm => mditm.mandays_categories_id == manday_category_id && mditm.pex_id == pex_id).id;
}

What I have tried:

If the GridView is not altered at code runtime, the FindControl works.