Why does the brightness go down when unplugging the charger?

Hello! KDE plasma wayland session on fedora 37 here. I noticed that the brightness (even if the indicator always shows 100%) is “less bright” with the charger unplugged, and brighter with the charger plugged in.
the strange thing is that this does not happen instantly, when I unplug it it slowly lowers the brightness, almost imperceptibly, and in about 4~5 seconds it stops at a lower brightness. the same happens when I plug it back.
am I the only one with this problem? googling it returned only unrelated issues.

(note, in settings > energy saving the option to change screen brightness on battery power is unchecked)

thanks in advance!