This is a current limitation of world-space UI Toolkit as documented here:
https://docs.unity3d.com/6000.2/Documentation/Manual/UIE-masking.html
"arbitrary shape masking is currently not supported in World Space UI"
As of today, overflow:hidden in world-space always uses a rectangle clip. Rouded masking shapes requires the use of the stencil buffer which may interact with other URP/HDRP features.
However, world-space masking is a planned feature.