If you are having problems with the Note 9 want to be in "night mode" even when it is turned off, I may have a solution. If you have installed and used Google Books, this may be the culprit. When you read a book, Google Book ask of you want to turn on the "Night Light" mode. This is similar to the "Blue Light Filter" of the Note 9. The problem is that once the "Night Light" is on, it can't be turned off. There is no setting in Google Book to modify the "Night Light" feature either. When the "Blue Light Filter" comes on, it becomes too dark to read. This is the same problem that the Galaxy 8 had and I'm surprised developers haven't fixed it.
Here's how to solve your problem. Uninstall Google Books. Restart your phone. Install Google Books and begin reading. When asked if you wish to turn on the "Night Light" while reading, click disable. Remember you need to read Google Books in a low light or night time environment in order for "Night Light" to be offered.