To reproduce: 
1. Add RadPivotGrid on the form and populate it with data 
2. Add 2 buttons to export to xls and xlsx formats 
3. Run the form and export to xls format 
4. Minimize the form. You will see that one of row is disappeared
5. Repeat the same action and you will see that a second row is disappeared. 

The issue if observed with both exporter providers (PivotGridSpreadExport/PivotExportToExcelML) and export with visual settings. 

Workaround: 
In the handler of the SizeChanged event clear the cached elements: 
void Form1_SizeChanged(object sender, EventArgs e)
{
    this.radPivotGrid1.PivotGridElement.PivotRowsContainer.ElementProvider.ClearCache(); 
}