This may be an issue with Brave, as I believe that the task manager collapses the new window into the pin when the window class matches, so it may be that Brave is changing its window class.
Maybe if you open a new Brave window where it isn’t folded into the pin, and then pin that window as the shortcut might fix it? I don’t use pins so I’m a bit unfamiliar with the way it works, but I hope this helps!