The logic is that if the deck was imported in the old format, then it will export in the old format... if possible. If you want to force exporting in the new format, just add some text to side 4 for a single card. You can delete the column after the export.
Thanks I was able to force it by adding notes to one of the cards. I don't see any obvious way to add another text side (side 4) within the app itself, though.