But if your master passphrase is only used for LastPass (as is exactly my case -- I've never used it elsewhere), how can it can be credential stuffing? Or was the password breached from LastPass itself in the past? That's possible, but then it doesn't jell with people having this same issue with accounts created in November 2021.

As far as I can tell, the "Someone just used your master password to try to log in to your account from a device or location we didn't recognize" email we all received was only sent when a correct password was used. Incorrect passwords did not trigger that email.

But yes, it would be great to learn that those emails were sent incorrectly i.e. it was a false positive (i.e. someone tried to login with a wrong password, but the email above was sent anyway). It's still not great that the wrong kind of email was sent, but that would be a low level bug.