This is not unusual for AA, I redeem on a regular basis with them and they change award availability very drastically from one day to another (don't be surprised if availability reappears) . BA would just follow that.
One of the important factors here is that you are flying between two AA hubs (DFW and MIA), since last year AA has been making it difficult to find availability on direct flights out of a hub. See this thread on the AA forum: More award availability restricted by married segments / connections

I haven't checked but I wouldn't be surprised you'd find plenty of availability on a XXX-DFW-MIA itinerary, yet if you search for just DFW-MIA on that same flight it would not show as available for an award.

It's infuriating really.