Was the new Excel document opened and checked after it was written to? I can successfully encode the file as UTF-8, check in console fine, but when Excel opens the document characters including the Hebrew text used in the test are gibberish. Notepad++ correctly displays it and if I use Notepad++ to encode it to UTF-8-BOM I can re-open the spreadsheet and it will display correctly.

Adding:
worksheet.getCell('A1').value = "\ufeff" + worksheet.getCell('A1').value;

Right before the writeFile works, also if this same file is then read using readFile \ufeff is undetectable - unsure if this is something worth adding as BOM isn't recommended but limitations in viewability of UTF-8 characters when opening actual Excel files for non-technical or data visualization may be helpful.