Comment 3 for bug 1768834

Update: The problem persists after re-installing ubuntu-mate 18.04.

According to
https://ubuntu-mate.community/t/lockscreen-bypassed-by-connecting-external-monitor-on-hdmi/16707/11
it seems that other users experience the same or a similar bug:
https://bugs.launchpad.net/ubuntu-mate/+bug/1769278