I don’t think there’s a bug to report here. I think the issue is there’s more green than you think. The orange area is 50/50 red and green which makes sense. Looks like your grass is populating about half as much in that area. Likewise where it’s appearing on the blue.
I think the issue is that if you want it excluded from the other areas, in your landscape material you should SUBTRACT the samples for blue and red before passing it into your landscape grass node. That should make it populate grass only on 100% green.