The new hair shading model seems to behave oddly when viewed specifically from the shadow side of stationary lights.
The current scene setup is being lit with either a stationary directional light, or a moveable directional light.
With a moveable light, hair shading works as expected: 
However, when using a stationary directional light, the shadow side becomes really bright. With stationary pointlights, this behavior only occurs after a lighting build:
I’ve noticed this is the case for both directional and point lights. I’m using the same tangent inputs as the hair shader in the latest ContentExamples, but similar behavior also occurs with a totally blank hair lighting model shader with just a base color and roughness input:
Moveable Light:
Stationary light:
I’m not sure if this will help, but here’s the hair mesh with the Default Lit lighting model to give an idea of what the vertex normals and topology is like:
If there’s something I missed about the usage of this shader, let me know. Thanks!