From some Googling, it does sound as though some people get confused with the way FCP
handles stereo, resulting in this kind of problem, though I haven't fully grasped it.
I'm wondering if the problem might arise in the course of adding further audio
tracks for foley, dialogue, etc., and getting in a muddle with the track linking/panning.
At any rate the video editor needs to keep a close eye on how the panning for the various
audio tracks is set.
Perhaps you could explain that you've suffered this
problem in the past, and could you check the video before they release it.