NPOI Write Corrupts File - Bare Ampersands - excel

Question

Using NPOI 2.1.3.1, I am trying to read an existing Excel (*.xlsx) workbook, modify it, and then write it back to the original file. After reading various threads (including this one), I still cannot find a solution to the problem I'm having.
When I write the file to disk and then try to open it again in Excel, I get the following error:
We found a problem with some content in (filename. Do you want us to
try to recover as much as we can? If you trust the source of this
workbook, click Yes.
Clicking "Yes" fixes various problems in the Excel file, after which I see the following report of the fixes performed:
Replaced Part: /xl/worksheets/sheet3.xml part with XML error. Illegal
name character. Line 3, column 3891168.
Replaced Part: /xl/worksheets/sheet19.xml part with XML error. Illegal name
character. Line 1, column 699903.
Removed Records: Formula from /xl/calcChain.xml part (Calculation properties)
I unzipped the *.xlsx file and found the sheets mentioned and discovered that the character it was referring to is a bare ampersand (&) that was not written as "&" in the XML. The original does use "&", but the file NPOI wrote does not. I have no idea what the issue is with the formula (third issue).
Here is a complete program that reproduces this issue every single time with the workbook I'm using, with the file name removed:
using System.IO;
using NPOI.XSSF.UserModel;
namespace NpoiTest
{
public sealed class NpoiTest
{
public static void Main(string[] args)
{
XSSFWorkbook workbook;
using (FileStream file = new FileStream(#"C:\Path\To\File.xlsx", FileMode.Open, FileAccess.Read))
{
workbook = new XSSFWorkbook(file);
}
using (FileStream file = new FileStream(#"C:\Path\To\File.xlsx", FileMode.Create, FileAccess.Write, FileShare.ReadWrite))
{
workbook.Write(file);
}
}
}
}
As a test, I wrote pretty much the same program using Apache POI, to see if it was just a universal problem with my workbook, and the result was that POI didn't have any problems.
Here is the complete program:
package poitest;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
public class PoiTest
{
public static void main(String[] args)
{
XSSFWorkbook workbook;
try (FileInputStream file = new FileInputStream(new File("C:\\Path\\To\\File.xlsx")))
{
workbook = new XSSFWorkbook(file);
}
catch (IOException e)
{
System.out.println(e.getMessage());
return;
}
try (FileOutputStream out = new FileOutputStream(new File("C:\\Path\\To\\File.xlsx")))
{
workbook.write(out);
}
catch (IOException e)
{
System.out.println(e.getMessage());
}
}
}
So the question is why is NPOI leaving the bare ampersands? Is this just a bug in NPOI?