Volumetric Lights in the shadow background

Hi, currently I am trying to achieve this shadows background that happening within the mesh in volumetric light scatter example that three.js have but I couldn’t achieve it… so what am I doing wrong here would like to get help thank you!

Currently it is visible to mine in a top view but below view or side view I couldn’t get it right.. I don’t feel the shadow presence behind its back.

https://codesandbox.io/p/sandbox/ckzrlz