Why do brightness keys change both displays when they are mirrored?

In a mirrored desktop, the pointer is visible on both screens. Screen with the mouse pointer therefore identifies the mirrored desktop rather than which physical screen you are looking at. BetterDisplay includes the selected source’s mirror targets when choosing displays for brightness-key adjustments.

Use the intended display’s own brightness slider when you want an explicit adjustment. If you want keyboard control to skip a particular display, turn off Keyboard control affects this display in that display’s settings. Check group synchronization as well if changes still propagate between displays.

If your aim is to use only the external monitor while keeping the MacBook open, soft-disconnect the built-in display instead of leaving it mirrored at zero brightness. This removes it from the desktop layout and avoids ordinary brightness keys lighting it again. Supported connection management and its safeguards still apply.

See Target Displays for mirror and stream targeting, and Display Connections for platform requirements and recovery.