I have noticed this behavior in Cycles hair rendering:
This is a close up of a some hair strands. What is causing the sudden changes of side of the “glint” in the hair strands? They are marked with red arrows in the image above.
I did not find information about this kind of behavior in the publications about hair bsdf. Nor did I find in cycles code where such a behavior could stem from (I am using Blender 2.80).