Ground Level Renders Dark in Fog - Halloween Forest

I have been trying to resolve an issue with my renders of the Halloween Forest.

It uses the Matte Fog and Ground Fog effects in the Environment Options. When I render it with out-of-the-box render settings, the area above and below ground level render at different brightness, creating a start delineation in the fog (see attached image). If I turn off the matte fog, the dark dilineation goes away. The ground fog does not seem to contribute to the problem.

Is there a way to fix this issue by adjusting the enviornment settings without losing the matte fog?