This preview will never show the customers data from custom export, that is a preview from hard coded data in the product and cannot be changed. It is just to check the layout.
You will need to save the export and run the data as an export type in export expenses section.
Create your custom export (preview to check the layout which will show Harry Potter - Test Data)
Save and name the export (for example data1) when you are happy with the preview
Within Import/Export - Export Expenses choose the export format data1 and then export.