Light probes giving unpredictable results

I've got a scene with a track.


There are no lights beyond the HDR skybox

I've created a script that places light probes along the track to create a volume extending either side.



However, as the vehicle moves along the track, incorrect lighting is applied, as shown here:



It's easier to see in a video:

I've tried increasing the light probe density all the way up to 1 slice/meter, but it doesn't help, just makes the transitions sharper/more frequent.

My lightmap for the terrain is has a _little_ noise but it's quite smooth, plus light probes appear to update at the start of the bake process, so I'm not sure they even depend upon the result?

Placing a large number of spheres shows the inconsistency:


What am I doing wrong? Why am I getting light where I shouldn't and vice-versa?

[Side note: Is there anyway to just recalculate light probes without doing the whole expensive bake process? It would make testing considerably faster]

Edit: I noticed when clicking on some of my indicator spheres that it would use a probe a log way off in preference to local ones if it was near the top of the shape. I tried adding a complete set of matching probes at y=100, and even spreading probes evenly across the terrain surface.

Neither helped.