Looks like a culling / rendering issue, though it could be distance field shadows too or exclusively. There’s different ways of culling, and if the foliage is being grouped for culling, then it would appear to be the problem as the shimmering pieces are spread out in different areas. Try increasing number of instances of foliage per group in culling settings, so it allows for more foliage before culling is initiated. And try changing the distance settings for culling.
What distance field shadows are being used in the scene? Only the directional light’s or the skylight DF ambient occlusion too?