Unfortunately, the AA IT system is built in layers over the old core of Sabre. Prior to bankruptcy, HP was to develop a new “Jetstar” PSS, but with bankruptcy and the AA - US merger it was decided to keep Sabre.
So: AA often does an atrocious job when ticketed passengers are subject to schedule (or equipment) changes, and all of us should “ride herd” on our tickets to prevent this kind of thing from happening.
When these things do occur, AA is usually pretty good at reaccommodating us - but it can be quite inconvenient. If you’d shown up three hours after your intended BA flight (not a BA “codeshare”, as AA awards are not booked as codeshares, but a BA flight operated by BA) had departed, AA would have rebooked you on the next flights with available seats to your destination, possibly prioritizing AA (regardless of award availability), and probably have worked out reaccommodation on BA, it’s joint venture partner across the Atlantic. That of course means you might have lost aone or more days of travel. (You might have been reaccommodated on the nonstop LAX-LHR flight as well, but that might have been challenging in First.)
When you fly BA as an award flight, BA charges you what they used to call fuel charges but now merely call “carrier imposed fees”. And if you fly on the way back originating in the U.K. on any airline you’re also charged the Airline Passenger Duty. See
Carrier Imposed Surcharge / YQ: AA award on BA / British (& IB) & avoiding it
and
UK APD / Air Passenger Duty charged for UK departures (Master Thread)
Next time do what we do: check your flights periodically. AA has seasonal schedules and other changes, and with the grounding of the MAX 8s has been forced to cancel ~115 daily flights. And if flights go awry, as others say, be “helpful” by checking available alternatives (that’s where ExpertFlyer or TripIt Pro come in handy) and asking for improved routing.