Bugs like this are why Apple should allow downgrading major releases, at the very least for a month after release. I upgraded immediately to iOS 14 upon release and after a couple days noticed this problem sending emails from my Gmail and iCloud aliases. I only caught it when a work contact replied to my message, and I saw the wrong address in my quoted text of their reply! I use email heavily for work on the go so this would have been a major disruption, but thankfully I could downgrade before Apple stopped signing iOS 13. I'm lucky I caught it in time, but everyone should have the option to downgrade for longer than the current 1 week.