In the endless list of known AAF export bugs I think this one's new:
Random stereo files, on the same track as others, are mono in the AAF.
So still two tracks, but with identical information. I think only the left or right channel is exported for both channels.
That's a real nasty one, as many people will never notice...
I have no idea why that happens. Just music tracks on a stereo track, 24bit wav, and some of them turned mono.
Again, other clips on the same track have remained stereo so it's a mystery.