Comment 28 for bug 1971434

I was having the very same problem.

The only (partial) workaround that worked for me was to delete the default "Super + L" lock-screen shortcut and replace it with a custom one using the same key combination and setting as command:

bash -c 'sleep 0.1 && xdg-screensaver lock && xset dpms force off'

This is based on these answers:
- Screen turns on automatically (xset dpms force off): https://unix.stackexchange.com/a/720365
- Setting custom shortcuts: https://help.ubuntu.com/stable/ubuntu-help/keyboard-shortcuts-set.html.en

Maybe the fact that a delay is necessary before of calling the `xset dpms force off` command might give some hint about why the bug might occur in the first place.

Now, the only missing part would be how to automate this same command after a given inactivity time, in a not too clunky way ...

Regards.