Thanks for the new set of files.
After an initial test, I am able to observe the issue as you mentioned by using your sample code with your template SpreadsheetML file. I found SpreadsheetML file when saved to XLSX file format via Aspose.Cells APIs, it contains protection settings applied. The worksheet in the original SpreadsheetML file is not protected but the output XLSX file is protected.
var loadOptions = new LoadOptions( LoadFormat.SpreadsheetML );
var workbook = new Workbook( @"InputFiles/SpreadsheetML.xml", loadOptions );
workbook.Save( @"OutputFiles/AsposeProtectionTest.xlsx", SaveFormat.Xlsx );
I have logged a ticket with an id “CELLSNET-45847” for your issue. We will look into it soon.
Once we have an update on it, we will let you know here.