Screen energy saving doesn't work if the Power Management service is running. No dimming, no switch off. If I leave the system at the login screen, the screen does switch off. Also, the command "xset dpms force off" works fine. I really would like to have Power Management running since without Suspend doesn't work. I have an NVIDIA card and is using the proprietart driver but I had the same problem with Nouveau.

Since Plasma 5 is still pretty buggy I suspect that this is just another bug that will be fixed eventually but perhaps there is some workaround in the mean time.