System Information
Operating system: Linux-5.10.0-4-amd64-x86_64-with-glibc2.31 64 Bits
Graphics card: TITAN V/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.56

Blender Version
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-03-22 15:27, hash: rBfb1265c5cf24
Worked: Don't think it ever worked

Short description of error

Changes in enabled Cycles compute devices are not detected as changes in user preferences. hence preferences auto-save does not happen.

Exact steps for others to reproduce the error

1. Go to User Preferences -> System -> Cycles Render Devices
2. Toggle state of the checkbox next to the render device
3. Quit Blender
4. Notice that there is no "Writing userprefs" message
5. Open Blender, and sure enough the changes are not saved

Note that it is important to only change the state of the checkbox. If any other setting is changed then auto-save happens correctly. For example, compute type change (None, CUDA, OpenCL) is properly detected as change in user preferences. Is really only the CyclesRenderDevice.use which is not detected.

@Julian Eisel (Severin) can provide a bit more details on exact reason why this is failing.