I'm running Ubuntu 18.04, upgraded from 17.10
When I try to suspend using the Ubuntu menu by holding alt button and clicking the pause icon the screen will briefly switch off and then turn back on again showing the login screen.
When I log in with my password the machine suspends properly (you can hear the fans stop, etc), but when I wake it back up it's already logged and does not require a password at all.
Note: if I type
systemctl suspend in the terminal I get the correct behaviour, which immediately suspends and upon being woken up will require login.