@araniSAN keep in mind things found in flags aren’t always active or meant to work. Some if it is experimental features, some are just still “there” from chromium but has no impact on Brave, etc. To my knowledge, immersive fullscreen has never worked on Brave. I am saying that because of topics like Immersive Fullscreen Toolbar and comments in the post over at https://www.reddit.com/r/chrome/comments/cszzol/autohide_toolbar_in_fullscreenmac/
What I’ll ask you for the moment is if things work well if you no longer keep that flag active. If so, that’s your solution for now.
In the meanwhile, I will go ahead to ask @Mattches if he knows any more on this and if there are any plans to build on it for people, since it’s been something that people have kind of been wanting for years now.