I don't know enough about it to answer the question, but it may be worth noting that the "shadow" from environmental lighting is just a dark area in the lightmap, not an explicit "shadow" texture applied to otherwise-lit stuff. In Halo 1 (and in all of Bungie's Halo games), environmental lighting of characters is handled by checking the lightmap at a single point and applying that lighting to the entire character.

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.

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