Gmail keeps editing mails. They have a concept of "dynamic emails" people can send now. Like if you get a mail notification about something in Docs, they will keep updating the mail in your inbox together with modified / added comments in the document.
Absolutely bonkers.
"Because of the dynamic nature of AMP messages, the content displayed in Gmail messages can change as time passes." https://support.google.com/a/answer/9709409?hl=en