My issue is the same as Using the set default browser button or manual kde setting for default browser doesn't work, but I see many unsolved issues with the default browser pop-up showing constantly.
Could somebody point me to the code that’s responsible for detecting the default browser?
I think the problem might be that on a Linux system you can have many desktop toolkits installed (like KDE and Gnome) at the same time (they are pulled in for some programs), and Brave isn’t necessarily looking at the default app settings for the toolkit that’s really used by the users Desktop.