I have the same question--I am certain I selected the fully refundable option when I found an itinerary I wanted at what seemed like an amazing price (compared to when I checked earlier in the week on AA and Delta). I haven't flown AA in several years because I used to live in ATL and almost always flew on Delta. Delta always says on the ticket if the fare is refundable and I assumed AA would do the same, but no. After searching and searching, I found my fare basis code:
NLW5DHPV

I think I figured out that NLW indicates low season, weekday. Not sure what the 5D is. H is the booking class of the first leg of the trip, P is the booking class on the 2nd leg of the trip. I don't know what V stands for.

Anyone have an idea what V stands for?

My itinerary is HSV-DFW & DFW-EZE. I cannot reproduce this fare basis code by making up dummy bookings. I guess that means the fare is no longer available.