Hi all, I wanted to point out something new in 4.27 that should help with the problem reported in this thread. There is a new cvar called
r.RayTracing.Geometry.InstancedStaticMeshes.CullAngle that enables solid-angle-based culling by default for ISMs, which includes foliage and grass. The default value is 2, which means that any instance with a bounding box that has a projected solid angle smaller than 2 degrees will be culled from RT effects such as shadows. Raising the number causes more instances to be culled. I tested the project linked above and the default culling behavior worked as expected. Hopefully this is helpful!