It still seems that the most likely answer might be that lastpass are incorrectly alerting that someone's correct passphrase had been used, and that the email is being triggered by a bug, or something like a login attempt using the wrong password from a suspicious IP.
The fact that lastpass support says that it means that the correct password was used doesn't mean it's true, the support staff might just be mistaken.
The fact that lastpass support says that it means that the correct password was used doesn't mean it's true, the support staff might just be mistaken.