Comment 4 for bug 1822637

Yes, Juju will internally figure out that a credential is invalid. Once this happens, Juju will not try this credential again unless a user tells it that the credential is ok to try again. This happens via 'update-credential' command. I agree that messaging could be better and I feel that it is covered by the bug you've previously filed - https://bugs.launchpad.net/bugs/1822117