Just want to double check on this, are you saying that you’re leaving this tab up and running? If so, do the notifications stop if you close the tab?
If you’ve changed it up in Mac’s settings, then it sounds like perhaps something to contact Mac on. It’s weird that they’d be having notifications show up if you have them disabled.
What about Gmail notifications or whichever email you’re using? And have you looked to make sure there’s no PWA that you have installed? If so, that would be separate than just Brave and you’d have to look for it and change notification settings.