In the latest version of the form, an additional field had been introduced. So I went to Data>Table, marked all records and replaced “n/d” with the desired content for that field. In the Data>Table view, this content is nicely showing and when using the URL https://kf.kobotoolbox.org/assets/.../submissions/?format=json one can also see that the new field now has values.
However, when downloading the data by “Export” on Data>Downloads, the whole column for that field is empty. Is this a bug?