Which one? He wrote multiple books on many topics. Unless "Mystici Corporis" referenced specific statements, we can't say that any of +Bellarmine's "teachings" were confirmed.
That heresy, by its very nature, separates the heretic from the Church.
"St. Robert Bellarmine, St. Alphonsus Liguori, Don Pietro Ballerini, Pope Gregory XVI, and many others, all teach that if a pope demonstrates himself to be a manifest heretic, i.e. a plainly manifested public formal heretic, he ceases by himself to be pope."
Kramer, Paul. To deceive the elect: The catholic doctrine on the question of a heretical Pope . Kindle Edition.