How to reduce the gray-line artifacts in three.js isosurface rendering of 3D texture example?

Question

the isosurface example in the three.js website shows gray-line artifacts over the surface, please see

https://threejs.org/examples/webgl2_materials_texture3d.html

the glsl shader source code is here

https://github.com/mrdoob/three.js/blob/dev/examples/jsm/shaders/VolumeShader.js#L183-L219

I am wondering if there is a way to reduce or remove such artifact? I tried changing some of the shader parameters, such as REFINEMENT_STEPS, dstep, but it seems there is no impact.