The official 2016 Q2 Release (2016.2.504) release will export Excel files with grid lines disabled.
The change is not intentional and will be reverted in the following hotfix and service pack releases.
As a workaround you can turn on the grid lines explicitly in the ExcelExport event:
.Events(e => e.ExcelExport(
e.workbook.sheets.showGridLines = true;
Apologies for the caused inconvenience.