Broken: 2.90, 5d2005c
Short description of error
When a hair emitter has non-uniform scaling applied, Cycles will render the hair particles incorrectly when they're part of an instanced BVH, for example when object motion blur is applied. Hair may abruptly change in width and/or shading.
Exact steps for others to reproduce the error
Open the attached Blender scene and render the animation.