When using the Landscape Grass Type, you are procedurally generating these meshes based on their assigned sampled layer. Since these meshes are instanced static meshes procedurally generated, they do not yet support static lighting.
Your best option, especially when creating large worlds with heavy foliage, is to use fully dynamic lighting in conjunction with Ray Traced Distance field shadows and Cascaded Shadow Maps to increase quality and performance.
We used this technique in our Open World GDC demo. I have provided documentation for Ray Traced Distance Field Soft Shadows, and some information on the Landscape Grass Type.
Using Landscape Grass Type
I have attempted to get your issue to occur on my end, but have not been able to reproduce it thus far. I have set up my test scene to have really dense foliage, but still cannot get the issue to appear.
Perhaps I am not doing something that same as you, but if you have other questions please let me know.
Thank you,