Workaround:

private IList<PivotGroupNode> collapsedRowsNodes;
private void customMenuItem_Click(object sender, EventArgs e)
{
    PivotGridSpreadExport spreadExport = new PivotGridSpreadExport(pivotGrid);
    spreadExport.ExportCompleted += spreadExport_ExportCompleted;
    spreadExport.ExportFlatData = true;

    this.collapsedRowsNodes = new List<PivotGroupNode>();
    PivotGridGroupTraverser rowTraverser = (PivotGridGroupTraverser)this.pivotGrid.PivotGridElement.RowScroller.Traverser.GetEnumerator();
    while (rowTraverser.MoveNext())
    {
        PivotGroupNode current = rowTraverser.Current;
        if (!current.Expanded)
        {
            this.collapsedRowsNodes.Add(current);
            current.Expanded = true;
        }
    }

    spreadExport.RunExport(@"..\..\export.xlsx", new SpreadExportRenderer());
}

private void spreadExport_ExportCompleted(object sender, EventArgs e)
{
    foreach (PivotGroupNode node in this.collapsedRowsNodes)
    {
        node.Expanded = false;
    }
}