I've also had this happen - twice now, both times when booking through AA. Last autumn I had an email saying my BA LHR-JFK flight (part of a longer itinerary) had been cancelled and I'd been rebooked on a later flight with AA. This didn't work for me and having checked EF to see that the flight was still listed with plenty of J availability I called AA who were quite insistent that the flight was indeed cancelled and they couldn't do anything. After some back and forth between them and the GGL line I finally managed to persuade AA that the flight did actually exist and had availability but the agent still needed to authorise it with a manager before reinstating me. I was a bit apprehensive when the day came but all was well in the end, especially as while in the CCR I noticed when checking the app that I'd been upgraded to F which was a pleasant surprise.

Then just last week it happened again on a booking with AA from ARN-LHR-JFK-SFO for May. Same story - I received an email saying the BA flight from ARN to LHR had allegedly been cancelled and I'd been rebooked on an earlier one. As before the AA agent was insistent that the flight really had been cancelled but after much back and forth and the involvement of a manager they once again managed to reinstate me, albeit with a new PNR for just that sector which in itself is mildly worrying.

They also suggested moving me onto a direct LHR-SFO flight so I could arrive earlier which was thoughtful of them but I politely declined, not mentioning that I'm keen to try the Chelsea lounge for the first time and fly F on the transcon again before it's phased out next year.

Anyway, it's annoying so I hope it doesn't become a regular occurrence. It would be interesting to find out exactly why as I don't recall it ever happening to me before these two occasions.