Referencing issue #630 and #632
This bug came with version 1.5.0 since one can show/hide worksheets.

Minimal working example:

var Excel = require('exceljs');
var workbook = new Excel.Workbook();
var sheet1 = workbook.addWorksheet('S1');
var sheet2 = workbook.addWorksheet('S2');

var fn = 'mwe.xlsx';
workbook.xlsx.writeFile(fn)
    .then(function () {
        console.log('Spreadsheet "' + fn + '" generated');
    });

outputs

As the documentation for the "new" (v1.5.0!) hide/show feature is not the best, providing a workaround would be nice. At least knowing how to access the show/hide API correctly would be nice until someone can fix it.