Note that master passwords were not compromised, the the warnings were triggered in error[1]. That's not to say that other concerns around security aren't valid.
[1] https://www.theverge.com/2021/12/28/22857485/lastpass-compro...
[1] https://www.theverge.com/2021/12/28/22857485/lastpass-compro...
Again, that it could be an attack vector is interesting and relevant, but different from "has been breached" per OP.