I would go ahead and first check and confirm that your Brave Ad Block Updater component is using the latest version. You can check this by going to brave://components and finding the Brave Ad Block Updater component:
Also worth checking which extensions you have installed in Brave as one of them may be conflicting with Shields, or trigger the Youtube ad block detector. On my end, I can use youtube (signed in or not) on both Windows 10 and macOS systems without seeing any ads.