Hello!
I had the same issue when rendering grass, I think I may have found a solution. I remade the alpha channel pic many times and nothing changed. This happened only for grass and not for leaves on trees. The difference between the two : leaves have black all around them while grass touched 3 sides of the alpha. So i cut a very small black outline around the alpha for definition and it worked. I think however small its allowed without compromising continuity will work fine.
