Describe the bug

Saving an .xlsb opened via SpreadsheetDocument throws an OpenXmlPackageException about an unexpected content type for /xl/styles.bin. This occurs even if no modifications are made; simply opening and calling Save() triggers the error for .xlsb. The same code path works for .xlsx and .xlsm.

Exception

DocumentFormat.OpenXml.Packaging.OpenXmlPackageException: 'The document cannot be opened because there is an invalid part with an unexpected content type. 
[Part Uri=/xl/styles.bin], 
[Content Type=application/vnd.ms-excel.styles], 
[Expected Content Type=application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml].'

A temporary workaround is to force-get all public properties of the SpreadsheetDocument instance before saving; after doing so, Save() succeeds.

Screenshots

N/A

To Reproduce

A self-contained, minimal repro (console app):

using DocumentFormat.OpenXml.Packaging;

var filePath = @"PathToXlsb\Book1.xlsb";
var fileBytes = File.ReadAllBytes(filePath);

var stream = new MemoryStream();
stream.Write(fileBytes, 0, fileBytes.Length);
stream.Position = 0;

using var spreadsheetDocument = SpreadsheetDocument.Open(stream, true);

// works with this solution
//foreach (var prop in spreadsheetDocument.GetType().GetProperties())
//{
//    try
//    {
//        _ = prop.GetValue(spreadsheetDocument);
//    }
//    catch { }
//}

// Throws exception
spreadsheetDocument.Save();

Steps

1. Open any .xlsb file with SpreadsheetDocument.Open(stream, true).
2. Immediately call spreadsheetDocument.Save().
3. Observe the OpenXmlPackageException.

Observed behavior

Accessing all public properties on SpreadsheetDocument prior to saving (reflection-based getter loop) makes the save succeed.

Expected behavior

Opening a valid .xlsb file, optionally modifying custom properties, and calling Save() should succeed without requiring any reflection-based workarounds.

Desktop (please complete the following information)

• OS: Windows 11 Home (26100.2033)
• Office version: N/A (repro does not require Office; opening/saving via SDK)
• .NET Target: .NET 8 and .NET 10 (both reproduce)
• DocumentFormat.OpenXml Version(s) tested: 3.3.0, 3.2.0, 3.1.1, 3.0.2, 3.0.1

Additional context

• Only happens with .xlsb. The same code works for .xlsx and .xlsm.
• The behavior cannot be reproduced with WordprocessingDocument or PresentationDocument for their respective formats.
• The workaround (calling getters for all public properties on SpreadsheetDocument) suggests a lazy-loading/initialization or part registration issue specific to .xlsb styles content types.