Imported from another thread where we were veering off-topic.
I would say that Councils do work on unanimity, but you haven’t accepted the Church definition of “unanimous”.
I would be interested to hear how to account for heresiarchs. Are they members of the Magisterium whilst within the Church? When condemned and excommunicated? In schism, are they members of the College of Bishops?
For a while in the 3rd-4th century, a significant plurality of the College taught that Christ is a creature of God and denied His divinity.
Up until the Council of Chalcedon, a significant plurality of the College taught that Christ had one nature, not two.
In the year 1054, a huge chunk of the College denied that the Holy Spirit proceeds through the Son, and they continue to teach against this.
The Council of Trent condemned a large number of heresies. Many bishops of the College clung to those Reformed propositions - but a universal magisterium must be unanimous?
After the First Vatican Council, many denied the Immaculate Conception and Papal Infallibility. Some were Catholic bishops! Infallible Magisterium, how?