After sleeping plasma does not display anything but the mouse cursor. That is, no lock screen. I can Ctrl+Alt+F1 to get a tty login.
Sometimes the display will return if I wait. Any ideas?
Has this issue been since original install?
Or did it start with a recent update?
Are you using X11 or Wayland?
If Wayland, the next time you log in, choose X11 then test.
(we do not have probs with Sleep mode on any of our machines running 15.6 - we only use X11)
The display configuration has been flaky since I set up the machine. I’m pushing the hardware to its limit with a dual monitor display at high resolution and refresh.
I switched from X11 to Wayland. I really don’t know what Wayland is. I recall when one had to hack to X11 config files to adjust resolution and refresh. There was no Wayland.
If you create a new user account, log in as that new user, then let the machine go to Sleep mode, then wake it up, do you see the same problem??
BTW …
Wayland is a protocol that apps use to communicate with a display server (for Wayland, is also the Compositor).
Basically, it’s intent is to gradually replace the X11 protocol.
We avoid Wayland because, for us, it’s still too buggy.
That’s how folks did it [waaaaaay] back in the old days.
Nowadays, for folks using KDE Plasma, if I want to change the screen resolution, I do NOT edit X11 config file, I use:
System Settings → Display and Monitor
… and click on the Resolution drop-down.
A plethora of choices, and no X11 config files to mess with 
.
Indeed, that I how I set things up. I recall when the KDE was in alpha, and fit on a few floppy disks.
As for trying Wayland; so far, so good. I haven’t had a problem since I switched from X11. But I also extended the time-out, so the machine hasn’t gone to sleep as frequently since switching.
I’ve seen some beguiling flakiness with the Plasma/KDE configuration in Leap 15.6. Somehow it has been changed without my intervention. I would love to investigate and understand how all of this works. Alas, I am dedicating my hours to mathematical physics.