If you have a customer who uses the Print and Send function to export the data and uses the function on General Ledgers, the excel file will be exported as .xml. If he tries to export from Customers or other tables, it is stored as .xlsx. We reproduced the same behavior on everything from Nav2017 to Business Central 14. We also have some custom tables that are exporting in random formats. We can not be expected to charge the customers per page to add custom export buttons for the "privilege" to export to .xlsx, and rather this should be fixed in the base app.