I know this is an old post, but I had a solution that ended up working for me.
The headers that are being used for the exports/samples are wrong. I had Product-URL as a header in my export and when I imported back into SquareSpace unedited, it couldn't find the missing column Product URL. I ended up removing the dash and it was able to import successfully. So verify that the missing header is matching. Hopefully this helps others!