STEPS TO REPRODUCE
1. make sure 'Raise maximum volume' option of plasma-pa applet is unchecked
2. open Audio KCM
the volume sliders can be used to set volume beyond 100%
maximum volume should be 100% when 'Raise maximum volume' is unchecked
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.25.80
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6
Graphics Platform: Wayland
Not exactly a bug, and fixing it requires reenginering where settings are stored for Audio & Volume things.
"Raise maximum volume" currently works on a per-applet basis, just like "Manually block sleep and screen locking" in Battery & Brightness applet. So naturally KCM in system settings doesn't know anything about that state. What's worse, the keyboard volume buttons are handled by one of the applets too. I'm saying "one of", because you could add multiple instances, e.g. on different screens or panels for convenience, and only one (pretty much random) of them will be responsible for those keys, as well as effective limits.
Basically requires fixing Bug 440846 first.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-pa/-/merge_requests/148
Git commit 5bdcba75652a8caa3f4e3302b7277fa36c7c6417 by Nate Graham, on behalf of Bharadwaj Raju.
Committed on 03/11/2022 at 23:54.
Pushed by ngraham into branch 'master'.
kcm: make volume sliders obey "Raise maximum volume" setting
M +1 -3 src/kcm/package/contents/ui/VolumeControlsConfig.qml
M +1 -1 src/kcm/package/contents/ui/VolumeSlider.qml
M +5 -1 src/kcm/package/contents/ui/main.qml