Dear friends, what are we to make of the old Catholic controvery over Pope Formosus. Stephen VI decreed that he was a false pope, then latter popes reversed Stephen’s decisions, only later to have Pope Sergius III ratify them. How do we reconcile this with papal infallibility. It seems to provide a good argument for Protestants
Papal condemnations of a particular PERSON apart from their doctrine does not fall under the prerogative of the Pope's infallibility, because it is a purely disciplinary matter. I can give you two other examples: In the Council of Chalced and the Council of Constantinople II: At Chalcedon, Ibas of Edessa, Paul of Samosata and Theodore of Mopsuestia were declared as personally Orthodox. At Constantinople II, their WRITINGS were condemned as heretical, known as the three Chapters controversy.
In the early 20th Centruy, Pope St. Pius X along with Pope Benedcit XV abrogated all former canons that were in force, whether by an ecumenical council or bulls or encyclicals, and gathered together all the relevant canons in a single codex calle the 1917 code of canon law.
So Popes can abrogate the disciplinary rulings of former Popes, because they do not have the Authroity to bind one another.