Excel file does not have default styling

Inge

I use Buttons to export table data to an .xlsx file, with this piece of code:

buttons: [
'excelHtml5'
]

What I expect is an Excel file with bold headers and auto sized column widths, the default styling according to this specification: https://datatables.net/reference/button/excelHtml5 But my file has no bold and all columns are too narrow for their contents.

I don't use customize or anything else to affect the styling.
Any idea where to look to fix this? Thanks!