geomipterrain lost lighting after flatten?

my geomipterrain has lighting from a directional light.
but if i set AutoFlattenMode or use the default flatten function from Panda, the terrain lost lighting - not become dark, but lighting become flat.

anyone with the same problem?

i could stand that the terrain is not flattened, but now i found flattening can solve a culling problem - blocks which are “nearly totally” out of frustum get culled, which is wrong. if the terrain is flattened, blockes don’t get culled by mistake.