With an upgrade to plasma 5.13 Keyboard indicator addon has got installed. When NumLock is activated and after deactivated the plasmoid still shows that Num keys are locked. However, when in the settings only CapsLock option is checked the behavior is correct: both lock and unlock are indicated properly.
I also use Arch Linux and can not confirm such behavior.
I have just figured out that on my Thinkpad x220, which doesn't have a physical NumLock button, the combination Fn+ScrLk used as a NumLock is generating an output in xev or in showkey when xserver is running just once. On the other laptpop with a physical NumLock the indicator works correctly. So I guess this is not a bug.