Landscape procedural grass lightmaps going wrong

I have a landscape with 5 layers of different materials, and procedural grass nodes in the landscape material to create 2 types of different grasses.
The problem now is, after lightmap baking for the whole level, there is this patch of grass geometries not baked correctly as the rest of the grass, it looks lighter in color and not receiving light or shadows
What could cause this problem? any thoughts about this?