That would make more sense than my convoluted "missing random language dialogue channel" theory.
I've just utterly failed to properly strip out the offending audio stream to see if there's any way I can examine it further, though (I was wondering if I could split it into the elemental streams, perhaps). Cutting it in mkvmerge, avidemux, Handbrake, ffmpeg or MP4Box fails in some way - normally turning the 5.1 section into a stereo track. And if you don't cut it, you're left detecting it as stereo since that's what it starts with.
Seems that anything ffmpeg-based (... so that's "everything"!) has problems in some way. And that assumes I get past all the "channel element is not allocated" errors... I thought I'd seek out mencoder as an alternative, but SMPlayer has its own problems with these files, and MediaCoder doesn't want to split files, so I suspect another dead end here.
I should take the hint and surrender gracefully!