I think I get what you mean there, but I'm not familiar with 'lightmaps'.

: So for example, if you cross through a narrow shadow while driving a vehicle,
: you don't see the shadow sweep across the vehicle... instead, the vehicle
: gets darker as a whole when its center is in the shadow, and gets brighter
: as a whole when its center is outside of the shadow.

That bit I get. Perhaps I'll do an experiment with a vehicle sometime, in some place where there's a narrow shadow. Then I can see it in action.

: Anyway, it sounds like the game maybe isn't properly sampling the lightmap
: for some characters.

Well initially I think it's all the characters.