I had this same issue. I was booked to fly from JNB to LHR on Thursday then connect to DFW on Friday (all on BA), then fly on to YVR on AA (which is a BA codeshare).

When I got up yesterday, I noticed the AA flight showed “cancelled” but I didn’t get any notification email. I called YouFirst and they said it looked like a system glitch, but didn’t know how to fix it. The agent assured me it would be taken care of and they would let me know when it was restored.

I never heard back from anyone, but the reservation reappeared in my record a few hours later. I just had to go in and re-select my seat (which was still open) and get my boarding pass when I got to LHR this morning.

Sounds like this has been a VERY common thing recently. Hopefully someone can find the root cause and get it fixed quickly. It’s not only panic-inducing and a pain for the flyer, it obviously is taking a lot of support staff time, as well. All for something that should ever happen between two closely-aligned partners.