Reset display brightness to factory defaults
Solution 1:
Default brightness is just ‘Automatically adjust brightness’ enabled. The brightness is constantly adjusted based on lighting conditions. If you disable this option, whatever the current brightness is that was automatically chosen is saved as the fixed brightness, so there is no default value.
Solution 2:
Reset the PRAM https://support.apple.com/en-us/ht204063 . This will reset display settings.
Kind regards!