This issue is more on lightdm, I've already explained this before so I won't explain further and rather I'll just put steps here, This problem is because the power settings are used from the gconf configuration of the user lightdm, and not the current user. Just test:
su - -s /bin/bash lightdm
gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type
If these returns
suspend then you can pass
nothing like below to change it and you should be good.
dbus-launch gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type nothing
just ignore any output you get,
reboot to take in effect, In my experience OS update 3 weeks ago reverted this setting which was in place since last year, but same worked again.
Also just verify
power settings after reboot. incase you still having issues.