How can I stop BetterDisplay from changing my Apple display’s auto brightness?

Turn off Enable native auto brightness management for the Apple display. This prevents BetterDisplay from managing its native ambient-light-based automatic brightness. It does not itself turn automatic brightness off in macOS.

Open Settings > Displays > [display] > Device Control > Device Control Settings… and disable Enable native auto brightness management. The option appears for Apple-capable displays. Then set automatic brightness as desired in macOS System Settings > Displays.

If adjusting brightness on one Apple display causes another Apple display’s automatic brightness to turn back on, disable this management option for all Apple displays in the setup, including the built-in display. Set the desired automatic-brightness state in System Settings afterward.

With management disabled, BetterDisplay also stops offering its native Auto Brightness toggle for that display. Features that normally enable or disable native auto brightness cannot manage it there. You can re-enable the option later if you want that behavior back.

See Device Control Settings for related brightness preferences.