System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 472.12

Blender Version
Broken: version: 2.93.5, branch: master, commit date: 2021-10-05 12:04, hash: rBa791bdabd0b2
Broken: version: 3.0.0 Beta, branch: master, commit date: 2021-11-02 22:38, hash: 11392829adfe

Short description of error
Cycles produces rendering artefacts for smoke simulation located inside another volumetric material, when the camera is located inside the second volumetric material. If the camera is moved outside the bounds of the second volumetric material, the rendering artefacts disappear.

Exact steps for others to reproduce the error

1. Select default cube and execute "Quick Smoke"
2. Duplicate default cube and scale to be larger than the first cube, such that the camera is contained within the duplicated cube.
3. Enter the shader editor and modify the material for for the second, larger cube:
   1. Delete the "Principled BSDF" shader.
   2. Add a "Principled Volume" shader node, and connect to the "Volume" connector of the "Material Output" node.
   3. Adjust the density of the volume so that original default cube can just be seen.
4. Run the smoke simulation (artefacts will vary depending on resolution or noise on/off).
5. [OPTIONAL] Add/amend lights and add background geometry (might help to add contrast and emphasise rendering artefacts)
6. [EDIT 15/11/2021 14:32 GMT] Set rendering device to GPU.
7. Go into camera-view and switch to cycles render in the view-port, or "Render Image" (F11).

Interestingly, the artefacts are more exaggerated in Cycles X (Blender 3.0 Beta).

Render artefacts with version 2.93.5:

Render artefacts with version 3.0 Beta:

Render artefacts disappear once camera is moved outside bounds of volumetric region:

Render artefacts with version 2.93.5: