I finally figured this out. The # is now used to mark columns that will auto-generate QR codes. But just removing the # doesn't fix the problem either. InDesign can remember that it existed there at one point. You have to completely delete that entire row in your spreadsheet and remake it.
Why they choose the number symbol as the symbol to wreck havoc in a data merge when a great deal of time columns in a merge are labeled as #1, #2, #3... is beyond me. So many of the world's data merge files have to be remade now. Apparently they couldn't afford the license for a ~ or something not related to numbered lists.
As you have been using this feature. Could you maybe help with the problem I have? I can get Data Merge to Create a V Card QR code but when I scan the QR Code it only shows the field names e.g. <first name> <last name> and not the actual first and last name from my excel file. Can you please help with the code?