It’s seems a bug to me. Before I was receiving 0.100 per ad and when I manually updated it to 1.18.75 all of a sudden started receiving only 0.010 per ad!
Same thing happened before as well and many people reported it
https://community.brave.com/t/how-much-you-are-getting-for-1-ad-i-am-getting-only-0-01-bat-for-1-ad