A possible fix was mentioned by stororokw in this forum thread.
Feel free to try it out as a workaround until this is addressed officially.
The dynamic shadow distance will change the falloff for back lighting on the hair material. The parameters listed for back lighting occlusion in the material graph didn't seem to have any effect.
The back lighting changes its falloff.
The back lighting is unaffected by the dynamic shadow distance.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-39061 in the post.