Specular reflections break when using Forward Shading

Forward Shading under 4.25.3 seems to break something in the Hair shading model. Default Lit seems to behave normally.

Reported and tested in 4.25.3(CL 13942748).

1. In Unreal 4.25.3, launch a fresh version of the DigitalHuman project in the Learn section.
2. Open the "DigitalHuman" map and observe that everything looks as expected.
3. Go to Preferences > Rendering and enable Forward Shading
   Once the shaders recompile, open the "DigitalHuman" map again and you will see the shading of the material is very flat and there appears to be almost no specular response when moving the camera view.

Result: Shading is very flat with no specular highlight response.

Expected: Expect it to look very similar to the original under Deferred Shading. So you would see clear bands of specular reflections tracing the edge of the hair as you move the camera.