This is the expected behavior as the token used in text above the field is parsed every time the form is opened and the value won't the saved. So if the approver open the form it will show the approver's display name. But when save to repository service task open the form to generate the pdf/tiff, it open it as anonymous user so it will show "Anonymous User". If you want the value to be fixed after approver approve the form, you can use a field with {/_currentuser_display} as default value instead. Then the value will be kept after approve.