Can you switch to DataGridView? This appears to be fixed, as
demonstrable by adding the following to my last post:

(add the bottom of the ctor)
       DataGridView dg = new DataGridView();
       dg.Dock = DockStyle.Fill;
       dg.DataSource = new SomeData[] { data };
       Controls.Add(dg);
       dg.BringToFront();

(add to the class)
   protected override void OnClosing(CancelEventArgs e) {
       e.Cancel = MessageBox.Show(this, data.Name + " : " +
data.Size, "Closing", MessageBoxButtons.OKCancel,
MessageBoxIcon.Question, MessageBoxDefaultButton.Button1) ==
DialogResult.Cancel;
   }

With DataGridView you see the new values in the message-box,
suggesting that updates and validation have occurred; with DataGrid
you get (as you posted) the old values.

Marc