Hi Metin. Actually all of the device settings are "preferences" and not saved with the scene, but should persist. This is by design. It is somewhat misleading in that they are presented in the scene settings. The other option was to put them in the addon preferences (where you enable addons) but we felt that was a bit hidden away in there.
Interested to hear your thoughts as we are reorganizing these in the near future.
Thanks for the explanation. I understand, the User Preferences are a bit hidden for non-experienced users, and less accessible than the Render tab. Maybe you could add "(Global setting)" to the device settings, to clarify it. If that's too long, "(Global)" should suffice, and you could then add extra info in the tooltips.
If the settings remains global and won't be saved along with a scene in future updates, then it would be great if the global switches are saved, so you won't have to activate "Use CPU" each time you start a new Blender session.
About Blender's tooltips, I would love to see each RPR function having a short explanation in the tooltips, so you won't have to open and search the manual each time you want to know more about a function. Maybe include the default setting / value for each function in the tooltips as well. That'd be very helpful and convenient.
Keep up the good work!