Although you are using a custom object, it works as a billboard. From the top viewport, you can see how the sun is hitting the items from different angles, and therefore the illumination changes.
To fix it, you can add some fake lights. As instance, in the attached image, i've created a omni light at north (90 degrees from the sun), Include->Illumination for grass objects and multiplier by 10.
For sure there must be other approachs, the trick is to make the illumination over the grass independent of light's angle.