Sandbox: https://codesandbox.io/s/issue-createportal-and-state-scene-pvtlsz?file=/src/App.jsx

Output:

According to the docs and type definitions, scene returned from useThree should be a THREE.Scene representing the scene. However, when this value is read within a component that has been dynamically parented to another object with createPortal, the scene value contains the parent object which could be a different type such as THREE.Group.

I would expect scene to always contain a scene so that components can modify scene-specific properties such as .background. I would only expect the scene value to change if the createPortal target was a different THREE.Scene.