C# setting ListView's Item's Subitem's text does not display

I am losing my mind a little bit here. When I initially build up a ListView, it works totally as expected:

ListViewItem listViewItem = listView.Items.Add(model.Id.ToString(), model.Name, model.Id.ToString());
listViewItem.SubItems.AddRange(new ListViewItem.ListViewSubItem[]
  {
    new ListViewItem.ListViewSubItem() {Name = "ArtNr", Text = model.ArticleNumber},
    new ListViewItem.ListViewSubItem() {Name = "Quantity", Text = "XXX"},
  });

I have 3 columns defined in the control, and they display, respecively, Name, ArticleNumber and "XXX", to begin with.

At a later stage, I have to replace "XXX" with actual data, and do the following:

foreach (ListViewItem listViewItem in listView.Items)
  {
    InvoiceLine invoiceLine = FindInvoiceLine(...);
    if (invoiceLine == null)
        continue;

    listViewItem.SubItems["Quantity"].Text = invoiceLine.Quantity.ToString();
  }

I can see the Text actually being set in the proper SubItem.Text, also if I check later, while referencing the listview through a different reference, i.e. I know I am looking at the right object. However GUI does not reflect the change, it still says "XXX" in the 3rd column.

What gives?!