This is an annoying issue. Please help!
Can’t reproduce and it seems totally unlikely as all of those commands are hardcoded in
Personally I’d suggest including your question in your description text, as all I see is “This is an annoying issue” under the “When I click shutdown, Lubuntu 18.04 Bionic Beaver” heading.
I would try shutdown via command
sudo shutdown -h now
and see if you get the same results (there are many other commands, but they’d likely have the same results). If they have the same behavior, I be suspicious of your hardware doing something quirky.
I’d next try other OSes (if not installed, tried using a ‘live’ session though some ‘live’ sessions can make testing power-down difficult) to see if they have the same results. The only box I’ve had that misbehaved, does it on all OSes; being a hardware quirk (failing/failed component possibly).
I just noticed a bug reported by a Ubuntu user reporting their system no longer halts power after OS shutdown, after a kernel upgrade (ie. box remains on with using power). They only have the issue on the latest kernel, so trying older kernels maybe worth trying, but you didn’t provide much detail (new install? old install? changes were made that caused a difference? etc)