It's hard to say. One way to check that it's a Forest Pack issue (rather than something shader related) is to test the scene again, but use Forest Tools to instantiate the scattered items to instances first. If the problem still exists they we can rule out a FP problem and check for other issues in the scene.
If you're able to, it might also be worth updating to V-Ray 5.2 to see if that helps.
Best,
Paul