Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.17
Broken: version: 2.80 (sub 74), branch: master (modified), commit date: 2019-07-01 14:26, hash: rBa6fe4d112ff4
Short description of error
In System Preferences for Cycles Render Devices, switching between None/CUDA/OpenCL marks the preferences as dirty for auto-save, however changing the specific device choices within say CUDA does not. So if your saved prefs have only your GPU enabled for CUDA and you check the CPU box as well, then this change will not be persistent.
Exact steps for others to reproduce the error
Open Preferences editor.
In the hamburger menu disable auto-save and click Save Preferences so we can watch the button to see the state of the dirty flag.
In System, Change Cycles Render Device from None to CUDA, and note that the Save Preferences button gets enabled indicating the prefs are now dirty.
Click Save Preferences to mark things clean again.
Change one or more of the device check-boxes for specific CUDA devices (enable the CPU device for example) and note that the Save Preferences button does not enable, indicating these changes are not marking preferences as needing to be saved.