Поправка.
Точка с запятой, видимо, здесь не проходит. Немного поправил команду, и все работает
bash -c "mate-screensaver-command -l;systemctl suspend"
I am using 18.04 fresh install. When I click suspend from the indicator applet in my tray area, on resume it does not prompt me with a lock screen. I have a screensaver and it is set to lock when active. This is a change in behavior from 16.04 and 17.10 for my laptop. I found an older thread from two years ago with a similar issue. From there I found a workaround that I can suspend and lock as expected if I press my laptop's physical power button while it is set to suspend. However, old habits ...