XSSFRichTextString.ApplyFont() works incorrectly

[Sergiy Stoyan]

NPOI Version

2.6.2

File Type

• XLSX

Upload the Excel File

It happens for any xlsx file.

Reproduce Steps

Run the following code:

 using (var xls = new XSSFWorkbook())
{
    ISheet sheet = xls.CreateSheet("test");

    var f1 = xls.CreateFont();
    f1.FontName = "Arial";
    f1.FontHeight = 18 * 20;
    f1.IsBold = false;
    f1.Color = IndexedColors.Red.Index;

    var f2 = xls.CreateFont();
    f2.FontName = "Arial";
    f2.FontHeight = 18 * 20;
    f2.IsBold = true;
    f2.Color = IndexedColors.Blue.Index;

    XSSFRichTextString s = new XSSFRichTextString("0123");
    s.ApplyFont(0, 2, f1);
    s.ApplyFont(2, 4, f2);

    var c = sheet.CreateRow(1).CreateCell(1);
    c.SetCellValue(s);

    using (var fileData = new FileStream(@"c:\test3.xlsx", FileMode.Create))
    {
        xls.Write(fileData, true);
    }
}

Issue Description

As follows from the code the cell string must be of the same font size and the substring [0-1] must be red. It is not the case.
The actual cell string appears:

It seems that the first font is not set at all and the default one is used.
BTW to work this around I had to use Append() which works as expected. But as my code works for both XSSF and HSSF, it would be nice to make XSSFRichTextString satisfy its interface requirements.