We have tested this issue with the following sample code and found, it generates corrupt Excel file.
We have logged this issue in our database for investigation and for a fix. Once, the issue is resolved or we have some other news for you, we will let you know asap.
This issue has been logged as
CELLSNET-45749 - Html String having multiple Fonts corrupts the output Excel file
Corrupt Output Excel File.zip (5.6 KB)
string strHTML = "<span style=\"font-family: 'Segoe UI', Arial, Helvetica, sans-serif; font-size: 12px;\">Text was entered here</span>";
Workbook wb = new Workbook();
Worksheet ws = wb.Worksheets;
Cell cell = ws.Cells["B2"];
cell.HtmlString = strHTML;