Why doesn't UIVertex.uv2 work?

The second UV set in a UIVertex is actually uv1, not uv2. The first set is uv0. Are you trying to work with the third UV set (which is called uv2) by mistake?

Found the reason, I think:

Description to AddVertex function is as such:

     // Summary:
         //     Add a single vertex to the stream.
         //
         // Parameters:
         //   position:
         //
         //   color:
         //
         //   uv0:
         //
         //   uv1:
         //
         //   normal:
         //
         //   tangent:
         //
         //   v:
         public void AddVert(UIVertex v);

So even though UIVertex contains uv2,3 (3rd and 4th set), the function itself isn't reading them. We need to use the other one I believe:

 AddVert(Vector3 position, Color32 color, Vector2 uv0, Vector2 uv1, Vector2 uv2, Vector2 uv3, Vector3 normal, Vector4 tangent);

I plan to test it soon.

[Update]

Yep, this extension function did the trick:

        public static void AddFull(this VertexHelper vh, UIVertex vert) => vh.AddVert(vert.position, vert.color, vert.uv0, vert.uv1, vert.uv2, vert.uv3, vert.normal, vert.tangent);