The shading is coming from the vertex normals, point all the vertex normals straight up and it will match the shading of the terrain better (provided you are aligning the grass meshes to the terrain normals). It still won’t match your unlit version but it will be consistent.
I’m not sure there is a convenient way of getting the grass to behave the way you want without implementing a new shading model with the forward renderer. Someone’s already done this though if you’re willing to compile the engine from source: [WIP] Anime/Toon Stylized Shading Model - Rendering - Unreal Engine Forums