Are you using some sort of blur settings on the camera?  If so, this could be causing the problem because the transmapped hair strands are getting mixed into the blur.  If you want to create a blurred background you can do one of two things:

1.  Render the image without any blur and then use the depth map to create the blur in Post

2. Do a 2 pass render one with the blur minus the hair, and one without the blur with only the hair and body and then composite them together in Post.