LWRP Trees Billboard not working as expected

I wanted to create a forest scene inside of a Unity Project using the Lightweight Rendering Pipeline (LWRP). But the tree billboards don't look right at all

You can clearly see the difference between the trees and the billboards. The billboards are just solid coloured shapes.

These are the material settings for the material used on both of the tree prefabs:

What am I doing wrong? How can I fix this?

Technincal details:
Unity Version 2019.1.9f1 Personal (DX11)
LWRP Package Version 5.16.1