Data Merge is ignoring the first field in the first row and inserting the next populated field in first row. Data merge manager preview shows the correct field.
Hi @adrianlambert,
Change the Layer order within your Group so the Month appears below your Data Merge Layer... I had to update the CSV file in the Data Merge panel but I think that's only because it didn't automatically link the file when I opened your Publisher document so you may not need to do that...