I tried to follow both methods, desktop restarts but fn button / multimedia buttons still do not work:
killall plasmashell
kstart5 plasmashell
or
kquitapp5 plasmashell
kstart5 plasmashell
from here
Also, restarting DE every time fn / multimedia stops working does not sound like a long-term solution 
Thx for support and ping me here if you need more debugging / more details about my system.
I spotted that in 100% cases this problem occurs is after the laptop sleep mode, but it might be not the first sleep after reboot…