If a master switchover is initiated and the specified server cannot be used as master, that it is not actively detected but will become apparent only as a side-effect of the attempt failing due to a timeout.

The test mysqlmon_switchover_bad_master tests this and the test reports:

Trying to do manual switchover to server4

{

    "errors": [

        {

            "detail": "MASTER_GTID_WAIT() timed out on slave 'server4'."

        }

    ]

}

So, the switchover fails, which is the expected outcome, but the fact that the server cannot serve as master should be detected immediately.