How to get billboard trees to receive realtime shadows?

I've created a basic Unity Terrain and added some trees and grass to it, using one SpeedTree and one standard Unity Tree. When the camera is close enough to show the tree geometry, the trees are correctly cast into shadow by the terrain. However, as the trees become billboards in the distance, they no longer receive shadows and look uniformly bright across the whole terrain. Grass, on the other hand, is working perfectly fine. I'd expect that I'd be able to get the billboard trees to render in shadow the same as the grass, but so far with no luck.

I have tried using the Advanced Terrain Shaders v2 but it makes no difference and I have to wonder if that asset isn't outdated since it's from 2012. I have been searching for solutions and trouble shooting this for more than a day and cannot figure out any way to get it work.

I'm on Windows using an NVIDIA GeForce GTX 1080 and my quality settings are set to Ultra High. I've turned off Lightmap Static for the terrain and trees as I'd like to have the sunlight position change over time.

Anyone know how to make this work? Do I need to edit/replace a shader, or is there some setting I'm overlooking?