Desired Lock Screen Behavior?

Just curious what the 18.04 lock screen behavior is supposed to be? Most of the time, pressing super-L will turn off the laptop screen entirely when locking, but a few times doing that brings me to a screen displaying the wallpaper but keeping the screen on until it times out.

Which is supposed to happen?

The desired behaviour is indeed turning off the screen.

But gnome-screensaver is known to have various issues - including your observation.

Its just waiting for a nice dev to investigate, fix and submit a patch or two to resolve.