Since media player controls were added to the KDE lock screen:

- fresh KDE login;
- hardware key on external keyboard and laptop key for play/pause media work
fine (media playing starts/stops as expected when pressing either hardware
key);
- lock screen (which recently had the media player controls added);
- don't touch the media player controls;
- unlock the screen;
- NOW the hardware key on external keyboard nor laptop key for play/pause media
no longer work (nothing happens when pressing either hardware key).