When your laptop battery gets low (< 20%), Gnome kicks you into power savings mode. Fine. But then it ignores your “idle dimming” settings, and starts cutting the screen brightness if you look away for just long enough to sneeze.
The workaround for this seems to be setting the idle brightness level to 100% — that way, when it does dim your screen, it doesn’t actually dim your screen:
gsettings set org.gnome.settings-daemon.plugins.power idle-brightness 100
See also
Thanks to u/190n for this fix: https://www.reddit.com/r/linuxquestions/comments/utle2w/ubuntu_2204_is_there_a_way_to_disable_screen/