Observado similar no Ubuntu 18.04 (baseado em xorg). Usando gsettings set org.gnome.settings-daemon.plugins.power ambient-enabled false resolveu o problema desligando o brilho automático.
Estou executando o Ubuntu 17.10 com o GNOME (Wayland) em um OLED do Lenovo X1 Yoga.
Acredito que esta instalação particular do Ubuntu começou com o Ubuntu 16.04 há vários anos. Eu tenho rodado o GNOME no sistema desde que foi instalado pela primeira vez. - Isso significa que eu poderia ter algum antigo controle deixado no meu sistema que eu simplesmente esqueci.
O problema: recebo muitas mensagens de erro do gnome-settings-daemon no meu diário:
feb 23 13:04:32 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:32 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
feb 23 13:04:33 kul pkexec[408567]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 843]
feb 23 13:04:33 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:33 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
feb 23 13:04:33 kul pkexec[408571]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 843]
feb 23 13:04:33 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:33 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
feb 23 13:04:34 kul pkexec[408575]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 758]
feb 23 13:04:34 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:34 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
feb 23 13:04:35 kul pkexec[408579]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 783]
feb 23 13:04:35 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:35 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
feb 23 13:04:35 kul pkexec[408585]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 800]
feb 23 13:04:35 kul org.gnome.SettingsDaemon.Power.desktop[1741]: Error executing command as another user: Not authorized
feb 23 13:04:35 kul org.gnome.SettingsDaemon.Power.desktop[1741]: This incident has been reported.
Desativei / habilitei o controle "Brilho automático" no painel "Configurações", mas parece que isso não muda nada.
Observado similar no Ubuntu 18.04 (baseado em xorg). Usando gsettings set org.gnome.settings-daemon.plugins.power ambient-enabled false resolveu o problema desligando o brilho automático.