I have a Dell laptop with KDE Neon (linux) and I try to make it close the laptop cover, turn off the screen (only turn off, not suspend) and lock the session (screen lock)
Well, I go into the Energy Saving section and there I have that with AC power (connected charger) the screen is blocked when closing the lid.
The thing is, it only locks, but the screen is still on even with the lid fully lowered. Just the same you can put the screen to turn off, but then the session is not blocked. And just let's put 1 option.
Here a screenshot:
I also thought that there may be a file in which these settings are defined. Maybe edit that file and tell it to also turn off the screen ¿?