SUMMARY
KDE brightness controls work correctly on OLED display until the brightness reaches zero. After this, it is impossible to increase the brightness again. The bug is triggered whenever KDE sets brightness to zero (via brightness key, brightness slider, or automatically when dimming the screen after inactivity). If the laptop is closed (sleep) and openen again (waking up), the display is coming back to normal.

This problem is also discussed in the KDE forum: https://forum.kde.org/viewtopic.php?f=17&t=175470

STEPS TO REPRODUCE
0. My laptop: Lenovo ThinkPad X1 Yoga Gen 7 (further OLED laptops were reported to show the same issue, see formum thread)
1. Start laptop
2. Use brightness key to decrease brightness to 0
3. Try to increase brightness again

Alternatively: wait on battery power until screen is disabled, then try to re-activate the display by any key.

OBSERVED RESULT
Screen stays dark.

EXPECTED RESULT
Screen lights back up.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.26 (Kernel 5.15.0-53-generic, X11)
KDE Plasma Version: 5.26.3
KDE Frameworks Version: 5.100.0
Qt Version: 5.15.7

ADDITIONAL INFORMATION
- Sending the laptop to sleep and waking it up again will restore the display (until dimmed again)
- The following commands also deactivate the display, but also succeed in activating it again:

 xrandr --output eDP-1 --brightness 0; sleep 1; xrandr --output eDP-1 --brightness 1 

- If the screen is disabled by xrandr --output eDP-1 --brightness 0, then it can also be re-activated by running xrandr --output eDP-1 --brightness 1 through a custom global shortcut. In contrast, KDE brightness controls do not work in this situation either.
- However, if the screen was disabled through KDE, then even xrandr --output eDP-1 --brightness 1 does not fix it.