On what device and model are you watching Apple TV content, when this happens?
The mentioned “Appearance“ seems to be a setting on a specific device. It may not apply to other devices. While such a setting may influence the brightness, I am not convinced that it is the cause of the issue, as most users wouldn’t have known of or touched that setting. Also it would influence all content, not just Apple TV alone.
The users that experience “too dark” seem to have in common that HDR is involved. If your device allows for customization of dynamic range, try to set for SDR, just to test if general visibility improves.