Brightness level on all Samsung phones is good... that ROM uses a lot of dark/black themes... so my guess is just the apps that are dark themed will appear lower in brightness.
However do check (in settings) your Screen mode.. if it is set to standard or Dynamic...check Brightness settings (in settings again, not toggles) to see if Auto is set and where the "line dot" is set... and further down check if you have "Auto adjust screen tone" checked.
But you have enough post counts to ask in that ROM thread for better answers. Good luck!
If you need help, check the FAQ first for whatever ROM you're on. Then ask questions.