Why does my screen brightness change when BetterDisplay starts?

BetterDisplay can restore saved combined brightness and software adjustments when it starts or the Mac wakes. If you adjusted brightness while the app was not running, restoring its saved value can make the screen unexpectedly dimmer or brighter.

Open Settings > Displays > Overview > Software XDR/HDR Upscaling and Dimming > Brightness Upscaling and Dimming Settings…. Under On startup & wake, choose Do not restore combined brightness and software adjustments if you want to stop this restoration. Choose Restore non-brightness adjustments only instead if you still want saved color adjustments restored without restoring brightness.

This is a shared policy, not a setting for just one display. Do not restore combined brightness and software adjustments also prevents saved DDC ranged/on/off adjustments from being restored and can clear their saved values. Restore non-brightness adjustments only leaves that DDC hardware restoration path available while excluding software/combined brightness restoration. Automatic brightness or another app may still change the level independently. If the issue is confined to an external monitor's hardware brightness, see DDC brightness restoration.

Compare the next app launch or wake after changing the policy. If the brightness still changes, the saved combined/software level is not the only thing to investigate. See brightness restoration settings for the separate full-dimming safeguards.