I am not 100% sure but this was the problem in my case. I had several screen share meetings with an Brave technician. We tried everything to solve the problem (about 6 hours) and the solution was using a different mouse.
I was using a “Roccat Tyon” gaming mouse with 1000hz and a “Roccat Taito” mousepad.
After using a different one “a basic microsoft office mouse” it worked without any problems.
He told me Brave Browser will only send an Ad notification if the Browser recognizes your PC is idling, but when your mouse is to sensitive this will never happen.
My PC is right next to my mouse (about 25cm away) with 7 fans running at 1200rpm… maybe the “micro vibrations” on my table are enough for forcing it to stay active. But like i said - i am not sure.
And i have tried everything before to fix this problem on my own. I’ve even reinstalled Windows just for Brave Browser 
Thats my old post - Brave Rewards are not displayed correctly