When I want to shutdown my computer, I have to click the power button in the task bar (not physical button on computer) twice until it does. The first time I choose whether I want to power off or restart then it just does nothing. I can continue using my computer, it doesn't hang like other people have experienced in previous versions. To get it to shutdown properly, I have to go to the task bar and press the button again then it properly does what I chose before with no dialog option.
I'm using the default Gnome desktop on 18.04 on a custom desktop computer I built. I recently installed a shell extension TopIcons Plus so I could get dropbox to properly show up and I noticed the issue. I removed it and it went away for a few restarts but now the issue is back. I didn't change any kernel settings as far as I know.