I’ve searched around for a solution for this matter but I did not find anything conclusive. Maybe this finding can help the bug to be fixed.
My environment setup has a movable directional light and stationary skylight.
From what I noticed there are times when the foliage in shaded areas are black (i.e. skylight color has no effect on it). It seems to be that the shading is dependant on only one of the foliage instance.
In the first image you can see the foliage is shaded by the skylight in the shadow areas.
Once i place another patch of grass on the other side of the lawn, the shading starts to turn black. If I removed this patch, the shading turns back to the original.
In one instance I’ve managed to find that the black shading is controlled by a single instance (the “alpha instance”).
If I managed to find this alpha instance and manually move it around using the select tool, the shading can be seen to be affected as I move that alpha instance around.
The workaround for me at the moment is to add multiple copies of the same foliage and swap to a new copy if I get hit by the black foliage problem. Once a copy hits the black shadow problem, I’ll just move to the next copy of the mesh and continue on painting.
Hopefully this gives some insight for how the problem can be fixed.