Matcap zebra line wiggles at the adjacent part of two continue spline surfaces

Hi all,

I am trying to use zebra line in matcap to check the surface smoothness of the model with multiple spline surfaces.

But there is always a wiggle of zebra lines at the adjacent part of two patches even the adjacent part is continue

The example below illustrates the issue.
The matcap zebra line works fine for single spline patch (ori. patch. 7x7 control points, u_order = 4, v_order = 4)

However, the wiggle appears when I duplicate the original patch (I did not move any control points) and delete the top two rows of control points of one patch (Patch A) and delete the bottom two rows of another patch (Patch B).

Does anyone know how does the wiggle come from? Or is there an alternative zebra shader that works for models with multiple spline patches

I gratefully appreciate any help.

Thank you