“I demonstrate in this volume that the proposition, that ‘a pope actually can fall into formal heresy’, is proximate to heresy, but it is not de fide; and the first Vatican Council, as the Gasser Relatio states quite unequivocally, did not intend to define on this point. On the other hand, the proposition that a pope can ‘teach false doctrines by way of the authentic papal Magisterium’, has always been generally accepted by theologians, even by Don Pietro Ballerini; and even after the definition on papal infallibility by the First Vatican Council, not only theologians, but even documents of the supreme magisterium admit that pronouncements of the authentic papal magisterium are not infallible, such as Lumen Gentium 25, which distinguishes between ex cathedra pronouncements which are infallible, and “the authentic magisterium of the Roman Pontiff, even when he is not speaking ‘ex cathedra’ – which is not infallible.”
Kramer, Paul. To Deceive the Elect: The Catholic Doctrine on the Question of a Heretical Pope (Kindle Locations 3514-3521). Kindle Edition.