As you may know, many office mice have thumb buttons (usually 2). With most browsers, these are mapped to the navigation buttons “back” and “forward” of the browser (similar to the way F5 is mapped to “refresh”), Brave does this too.
When I press the thumb buttons, Brave usually ignores it. Sometimes after closing and opening the browser again, it seems to work for a few seconds or sometimes even a bit more, then it goes back to ignoring it. It also rarely seems to work just out of thin air, and then goes back to immediately not working again.
I also tested it with the Chrome Browser and had no issues there.