So, have solved the ‘key’ issue by changing the metadata type to Decimal Number – i.e. something incompatible with the data in that column, which is alphanumeric.
Another interesting issue is that each time I have done a batch of records (between 3 and 10), one has not transferred the metadata. I cannot see anything wrong with the record. One time, the metadata ended up populating the csv file’s metadata. If you have any suggestions for why this might be happening, I would be glad to hear.