Transluenct Cast Shadows As Masked Not Working

Hello,

I’m aware of this UE4 feature to Cast Translucent Shadow as Masked:

However, every test I’ve done seems to not result in the desired effects.

This is my current setup for testing:

The documentation makes mention of “lighting rebuilt using Lightmass” but I couldn’t find a way to enable or disable “Lightmass” if that’s an option.

You can probably see what I’m trying to make, clouds with cast shadows. I’ve come up with a solution that would just have me syncing 2 planes up, one being one-sided masked material that casts the shadow, and the other being the cloud material that can be seen from below. That would give me the effect, but I still want to know why this specific cast shadows as masked isn’t working.

Is it because it’s not possible to use movable lighting/objects?

Any help is much appreciated.

Thank you,
-Metric