As can be seen in the picture, there is no accelerator in the "Lock screen" label to the "Ctrl+Alt+L" input. Instead, the accelerator is in the box itself, making it pretty unreliable and thus hardly usable. Additionally, the accelerator of the first item "Lock screen automatically" changed from L to s in Plasma 5.8. Reproducible: Always
That is kind of unexpected to have on the shortcut button - agreed.
Fixed in the QML rewrite.