Yes, I did exactly that, I selected the radio button for "Template" rather than "document" when working with these.

But! the good news is after much testing and torture, I FINALLY GOT IT TO WORK! The CSV file was the culprit.

The CSV file originated from a third party - I'm not sure how they are converting their data to the format, but instead of my usual downloading the file with a "save target as", I opted to OPEN it instead, it of course opens in the new edition of Excel, I then chose to Save As, and then close it and when it asks yet again if I want to save it, I actually go through the save process again - and THEN it worked. So now when I use this CSV file it is retained as the source data with my Word Merge Template main document - permanently.

Now when I need to update the document, as usual I overwrite the old CSV, but go through the round about saving process in Excel first.