This is a Dinamic Light issue.
Regardless of the skylight being set to 0, any intensity at all on the dynamic light during night time will cause some geometry that protrudes under the landscape to be lit.
A tried and true Fix All for this is to create a box as wide and long as the landscape and place it under it. I usually coat mine black and also disable shadow casting on it.
This perfectly prevents any light from ever getting to the foliage or any other mesh that was purposely sank into the landscape.
To work with large maps/worlds turning the light down to a really small number might be preferable to the box method.
This doesn’t always work, it depends on the post process and skylight settings as well.
Easy to tweak, hard to learn.