Why is DDC disabled for my Apple Cinema Display when another app can use it?

BetterDisplay treats Apple displays differently and can deliberately withhold DDC control to avoid brightness problems. Another app successfully sending DDC commands does not establish that the same control method is safe for the display across sleep and wake.

The relevant override is Settings > Displays > [display] > Device Control > Device Control Settings… > Treat as generic third-party (non-Apple) display. It appears for displays identified as Apple-made or otherwise Apple-capable. Enabling it allows the display to be handled as a generic device so DDC can be configured.

The app warns that this can cause permanent issues on some older Apple displays. Keep the default unless you have established that your particular display and connection can safely use DDC. A successful brightness change alone is not that proof. A display can still wake black or fail to respond afterward.

This override is not a general fix for DDC through docks, and it is not expected to appear for an ordinary third-party monitor. See DDC through adapters and docks and Device Control Settings for the separate connection and controller choices.