Have you tried to render the motion blur computing the velocity attribute with the Trail SOP? Perhaps the problem is that the hair object doesn't have a constant topology among the frames, making the computation of the motion blur from the mesh deformations impossible.
In any case, if you want to share a sample scene with this issue, I can take a look at it.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini