Glad it alerted you. I really like Apple’s built-in password management. I had an identity theft attempt (thwarted by the fact that I work in finance and got ahead of it, but scary nonetheless), and Apple’s password manager was like, “Uh…ya might want to change some of these ASAP”!
I’ve seen this in the past but it turned out to be because the password wasn’t in 1Password - when I migrated away from Keychain I changed a lot of passwords in 1Password but never bothered to update Keychain.