What if you tried removing those UseColumnTextForButtonValue statements and initialized whenever a new row is added?

private void InitializeParentGrid()
{
    parentGrid = new DataGridView
    {
        Dock = DockStyle.Fill,
        AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.Fill,
        RowHeadersVisible = false,
        AllowUserToAddRows = false,
        ColumnHeadersVisible = false // Hide column headers
    };
    var expandColumn = new DataGridViewButtonColumn
    {
        Name = "Expand",
        HeaderText = "",
        Width = 30, // Set a small width
        AutoSizeMode = DataGridViewAutoSizeColumnMode.None,
    };
    parentGrid.Columns.Add(expandColumn);

    var yearColumn = new DataGridViewTextBoxColumn
    {
        Name = "Year",
        HeaderText = "Year",
        DataPropertyName = "Year"
    };
    parentGrid.Columns.Add(yearColumn);

    groupBox1.Controls.Add(parentGrid);
    parentGrid.CellContentClick += ParentGrid_CellContentClick;
    parentGrid.RowsAdded += (sender, e) =>
    {
        var exp = parentGrid.Columns["Expand"].Index;
        if (parentGrid.Rows[e.RowIndex].Cells[exp] is DataGridViewButtonCell button)
        {
            button.Value = "▶";
        }
    };
    parentGrid.Rows.Add();
    parentGrid.Rows.Add();
    parentGrid.Rows.Add();
}

private void ParentGrid_CellContentClick(object? sender, DataGridViewCellEventArgs e)
{
    if (e.RowIndex == -1 && e.ColumnIndex == -1 ) return;
    if (parentGrid?[e.ColumnIndex, e.RowIndex] is DataGridViewButtonCell button)
    {
        button.Value = 
            Equals(button.Value, "▶")
            ? "▽"
            : "▶";
    }
}