“However, users receiving these warnings have stated that their passwords are unique to LastPass and not used elsewhere.”
LastPass users got alerts that someone had logged in with their master password from a strange location. LastPass called it credential stuffing. That explanation needs reused passwords, and these users say theirs were unique. Some changed the password and got another alert. Others couldn’t even delete their accounts. A password manager that answers with a PR line has already told you what you need to know.