Why does a fully dimmed display become visible again after wake?

BetterDisplay normally avoids restoring full dimming at startup or wake so a saved black screen does not leave you wondering whether the display is working. You can allow restoration, but that is different from saving a separate brightness preset for each docked or undocked arrangement.

Open Settings > Displays > Overview > Software XDR/HDR Upscaling and Dimming > Brightness Upscaling and Dimming Settings…. Under On startup & wake, choose Restore combined brightness and software adjustments, then enable Allow restore to full dimming if you want black-level settings restored.

Apple displays have an additional safeguard: Restore excessive dimming for Apple displays permits zero software brightness and combined settings whose hardware component is lower than the currently reported brightness. This option is off by default because very dark values saved during wake transitions may not represent your intended working brightness. Enable it only when you want that behavior.

These choices affect restoration of combined brightness and software adjustments. They do not create automatic 0% docked and 80% undocked presets. Mirroring and brightness synchronization can also affect which display is adjusted.

If your real goal is to use only an external monitor, automatic disconnection of the built-in display is usually a more direct fit than mirroring it and dimming it to black. Soft disconnection requires Pro and a compatible Mac. It removes the built-in desktop from use while the applicable external-display condition holds, with the documented recovery behavior.