I’m having the same issue. I have apparently received 36 notifications, but I have yet to actually see anything…
Other users say they pop up in the lower right. I have not seen this happen yet.
Edit: I want to add that I am running the latest version of Brave, using Windows 10, and have Focus Assist turned OFF.
Double Edit: Resolved this issue. See https://community.brave.com/t/brave-ads-windows-10-troubleshooting/56730