I had the same problem beginning a few days ago (ads suddenly stopped/windows 8.1). I fixed it by enabling the two “expirimental flags” below and then relaunching brave browser:
- brave://flags/#enable-system-notifications
- brave://flags/#brave-ads-custom-notifications
It may be that only one of these flags was necessary to fix my issue. But now I am getting notification ads again after several days of none.
If you are on windows 10 or above you may also want to open Windows Settings and search “notifications” and ensure that they are permitted for Brave.