FontName becomes OpenXmlUnknownElement

[andrewtsybulia]

Hi team
Describe the bug
After updating to version 3.1.0, I've encountered a bug. When I create a DocumentFormat.OpenXml.Spreadsheet.RunProperties with FontName as a child element, it changes the FontName to OpenXmlUnknownElement.

Screenshots

To Reproduce

using System.Linq;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;
using Xunit;

namespace SOME_NAME_SPACE;

public class ExcelConvertingHelperTests
{
    [Fact]
    public void BuildFont_Should_Return_RunProperties_With_FontName()
    {
        // Arrange
        var runProps = new RunProperties(new FontName());
        var spreadsheetDocument = SpreadsheetDocument.Create($"SampleWithRunProperties.xlsx", SpreadsheetDocumentType.Workbook);
        var workbookPart = spreadsheetDocument.AddWorkbookPart();
        var worksheetPart = workbookPart.AddNewPart<WorksheetPart>();
        worksheetPart.Worksheet = new Worksheet(new SheetData(new Row(new Cell(new InlineString(new Run(runProps))))));
        workbookPart.Workbook = new Workbook(new Sheets(new Sheet()));
        spreadsheetDocument.Save();

        // Act
        var result = worksheetPart.Worksheet.Descendants<RunProperties>().First().FirstChild;

        // Assert
        Assert.True(result is FontName);
        Assert.True(result is not OpenXmlUnknownElement);

        spreadsheetDocument.Dispose();
    }
}

1. Run the test
2. See error

Expected behavior
FontName becomes OpenXmlUnknownElement, I expect it to stay FontName.

Desktop (please complete the following information):

• OS: Windows 11 Enterprise 23H2
• .NET Target: .Net 8.0
• DocumentFormat.OpenXml Version: 3.1.0