Modeling of surface roughness in Ansys Q3D (2D solver)

In Q3D 12.0, solver provides two surface roughness model:

Hammerstad Model
Huray model

I used Hammerstad Model and doesn't experience problem. However, Huray model with its default parameters can generate strange insertion less response.

The strangeness is that IL of transmission line can become non-monotonic in frequency. Group delay also strangely bends up.

Any idea why this is happening? How to set the parameter in Huray model correctly?

Thanks,
Neo