Welcome to the Rogers Community Forums and congratulations on your first post!
I prefer a darker display so I have Night Mode on my Note 9 all the time but I can understand how this may not be ideal for all users.
Can you confirm how you enabled Night Mode? Was it via the Quick Settings dropdown menu or did you go into the Settings > Display > Night Mode route? If via the Quick Settings dropdown, please attempt the Settings route and see if that works.
If you have not already, please Restart your device and try to toggle Night Mode off again. I have had no issues toggling Night Mode on and off via the Quick Settings and Display Settings.
Are there any other Note 9 users in the Community running the Samsung One UI that cannot disable Night Mode?
We look forward to your response!