This keeps happening to me. I just checked my excluded sites (Brave Rewards) for the first time in a while and there were 17 sites added to the list that I never added. I made a post about this before in the the rewards support section and I never got any feedback, despite me bumping it. It has since closed (30 days of inactivity) so I’m making a new post here.
This is happening to me too. Just like OP, occasionally when I check Brave Rewards I find that some sites were excluded, despite me never having done so myself. My Brave version is:
Version 0.68.140 Chromium: 77.0.3865.90 (Official Build) (64-bit)