I set screen to switch off after 10 minutes of inactivity.
However, the screen just stays on.

Here is how Energy Saving is set:
Menu > Computer > System Settings > Hardware, Power Management > Energy Saving
check: Dim screen after: 9 min
check: Screen Energy Saving, Switch off after: 10 min

I am using KDE Plasma 5.10.5 on Fedora 26 Spin.
Hardware: Haswell Intel NUC Kit D54250WYK, Intel Core i5-4250U CPU @ 1.30GHz × 4, HD Graphics 5000
Screen off with Gnome worked on this hardware.

Update:

The screen switches off when Chrome browser is closed, but not when Chrome is open.
Other applications (Dolphin and Emacs) have not effect, and screen turns off as intended.

This may have something to do with KDEConnect https://community.kde.org/KDEConnect
https://forum.manjaro.org/t/kde-power-s ... ng/12207/6