For example if you login to your google account in regular tab. and use different google service in private tab and TOR tab. Google may able to link all other tab content to regular tab account. because it may not share Cookies but Google uses different tracking methods. so your Brave TOR Tab activity may track back your real identity. not happened with real Onion Tor Browser.
i use private window for untracking or temporary workings as creating mail or log-in any account different from mine. all in same profiled process ( i mean if you open more tabs or windows) with same profile, cookies and sessions shared. use new profile or private window to bypass it.
Cookies are one of old age trackers. now with use of Javascripts websites owners are use 50+ unique identifiers to track users. if you dont use random value genereators its most likely to track easily even if u use different profiles or seperate cookies containers.