Bug shader on Pro Builder Shape looks weird in small shape size

My environment:
windows 10 enterprise
Unity 2021.3.16.f1
ProBuilder 5.0.6

What I have done:
1.Open ProBuilder Window and click "New Shape".
2.Pick "Sphere" type and draw a sphere.
3.Set "Size" x,y,z to 0.05.
4.Drag any texture on this GameObject, let's say a pure black png image.Then you get a normal sphere like this:

5.Change Subdivisions to 4 or 5,then you'll get a sphere with a really weird shader compared with those whose Subdivisions are 1,2,3.



To make their shaders look more clearly,I change some of parameters and make a full comparison:


From the upper result, it seems that when the shape size become smaller, the Subdivisions need to be smaller as well to keep the sphere shader looks normal and realistic.

So is that phenomenon a bug?

I would really appreciate if anyone could help me out.