Hi there,
I’ve seen a good post on this earlier.
https://forums.unrealengine.com/t/foliage-has-a-shadow-noise-grain-how-to-fix-it/727330/36
It likely has something to do with virtual shadow maps and raytracing combined with soft shadows.
You can check it by setting your sun angle to 0 in your directional light.
Potential fix (for now) is to increase the rays per pixel on raytracing and messing around with the sun angle.
That worked for me at least.