Right, I've been investigating again. First, it wasn't obvious to me before, but clicking the "Lock exposure" box adds a couple of sliders to adjust exposure, one for RGB and one for infrared.
Normally when Vuescan starts up, it defaults to the last settings. This time it had somehow reset itself, so I had to re-create the last settings from memory. In the capture of the Color tab below, I missed one, with output colour space set at AdobeRGB rather than sRGB:
Eventually I was able to turn the exposure down enough to get a reasonable scan of this frame. The next dodgy one, however, looked like this (the cyan areas are out of gamut/over exposed):
Even with exposure turned down to 0.1 I wasn't able to get much improvement.
At this point, it seemed worth while to look at the frames by eye. I don't have a loupe, so I can't be sure, but I think 99% of that last frame is fine. One or two leafs may be badly over-exposed, if you're looking really closely. I did another frame with similar results (it was also yellow leafs).
So I'm still bamboozled!