By disabling the extension and using Block ads and trackers protection in Shields, I was able to block the ad in question. Shields with the general Block ads and trackers option enabled do not disable this ad. Ads on other sites seem to be block by fine by Shields and the extension.
As for the ad itselt – here is my screenshot of it:
The fact that technically the Shields icon will be on/enabled always produces confusion easily as generally disabled things turn into grey or disappear. I would suggest to remove Shields icon from address bar or turning it into grey if disabled completely generally of for site.
I would still stick to AdGuard as it allows blocking within operating systems for different devices, while Shields allows blocking only within Brave browser.