In Odoo 19, this behavior is normal because the exported field “Nom d'affichage du partenaire de la facture” corresponds to the technical field partner_id.display_name, which is a computed field and cannot be written to during import. Odoo allows exporting display values for reference, but during import it only accepts writable fields. That is why the header is not recognized when you try to re-import the same file. To fix this, replace that column with a column named Partner (technical field partner_id) and ensure the values match exactly the existing customer or supplier names in your database. For a more reliable import, especially if you have duplicate names, you can use the partner’s External ID instead of the display name.