If you are confident that you are correct, I would do the following:
- Try to make a new booking and screenshot the resulting fare, including the text that shows it is refundable.
- Get any other evidence you have, including the date / time you called AF and your receipt.
- Send a letter by post to Air France (with signature required) and give them 30 days to refund you
- If that does not work, file a lawsuit. I would just go to small claims court in LIT. There is also an EU procedure to file online.
I booked an AF fare recently, though, and specficially remember that "Flex" did not include free refunds. It only allowed to changes at a reduced cost.