Subsurface & light transmission - bad shadow?

Hey!

I’m using a SS shading model for a character, but when I turn “Transmission” on in the light settings I get really bad looking shadows.

I’ve uploaded 2 images with just one of my lights tuned on. One image is with transmission turned on, and one with it off. This is quite an extreme example.

Any suggestions as to what’s causing this?

Many thinks!