First of all, I am not sure whether this is a bug of KDE Plasma. But I have
entered into this situation several times before when I didn't pay much
attention to it.

When the Windows 7 guest of VirtualBox is in full screen mode, the openSUSE
Tumbleweed with KDE Plasma won't lock the screen when the laptop lid is closed.
The screen is locked as supposed if the guest is not in full screen mode. I
think the host OS should lock the screen when it is set to while the laptop lid
is closed.

This issue could always be reproduced.