Of what? Not your Rewards. If you’re speaking of Uphold, I think you’re jumping to the wrong conclusion. We had someone recently speak on this. Uphold has changed their site to log Users out of the site after 15 minutes of inactivity.
Brave won’t erase cookies or log you out of websites because you use VPN or anything. However, those sites can have your cookies expire if your details do change. That would be those sites, not the browser. And sites would do this to protect you.