The 1.35.100 (Chromium: 98.0.4758.87) browser release has introduced a bug for Brave Rewards estimated earnings, where those earnings can drop upon upgrading for a certain percentage of users. No BAT was actually lost and this is just a display error. The team is working on a hotfix for this now. Once that is available the display will return to normal. Thanks for your patience and sorry for the inconvenience.