The mesh looked unlit to me in the first picture, suggesting that it only receives light from a skylight, which is why I mentioned shadows. It seemed to me that the mesh was simply ignored by the directional light. Due to what you said in your post my presumption is that you could try setting the VHFM actor to movable (if that is at all possible) to ensure the directional light is considered for shading, since it doesn’t interact with the baked GI you need.
I did not take the time to recreate a similar scene, so I’m just poking in the dark here, my sincerest apologies.