see attached document, it was exported by ExcelExporter, and could not be open by Excel or Spreadsheet anymore.
to reproduce this file.
1.use spreadsheet and src to a empty xlsx.
2.in spreadsheet, in A1, input 'A'
3.drag the right-bottom corner to A2 or A3, it will copy A1 to A2 , and A3 as well.
4.use exporter to export this book.