Thanks for your reply! The grass is not casting any shadows in the screenshots. I disabled that feature I also tried switching off the “tangent space normal” option inside the material that I use for the foliage… and a ton of other stuff.
The shading issue seems to be caused by the way the mesh planes interact when they “catch” the light from the directional light source. The meshes that are not directly facing the directional light are rendered as really dark, whereas the ones that are turned directly toward the light are rendered as light. I am at a loss as to what to do to fix it.