Opening up the settings of the battery and powermanagement widget and unticking boxes for display timeout or others and hitting apply and reopening the settings shows them checked again. Changing the values only and hitting apply saves the settings and reopening shows the right settings. The config file seems to be written correctly. Though when unchecking the settings it removes all previously stored values. So it will initialize with default values on reopening.
Seems to be a bad combination of a default settings file shipped with the distribution as default in a xdg data dir folder.
Cannot confirm on Arch Linux, plasma 5.14 beta. Really it seems some bug with your distro.
Powermanagement should still be able to save settings even though xdg config dirs overwrites its default settings. So it is not a downstream bug.
Can you include this default settings from this distro?