Spreadsheet reports are exported to Excel format when sent via email, and also when 'Save As Excel' is used while viewing a report.
When you first open an Excel attachment it will show in Protected Mode where formula cells are shown blank. To exit Protected Mode and enable formulas, click the Enable Editing button at the top of the window:
This step is required each time a new attachment is opened.
It is possible to disable Protected View in Excel, but this step should be taken with caution.