Leo and all the other V2 popes are absolutely heretics. No doubt there. None.
The only “doubt” concerns their papal status. And that’s only because it’s a first-time-in-history situation.
Had the church previously had a pope-turned-heretic in office before…or a case of a heretic-prior-to-office-therefore-invalid-papacy, and given us historical guidelines, then V2 would’ve never happened. Or, it would’ve been ignored by so many more Catholics.
However, God has deemed that this situation is a novelty. The modernist heretics have used this to their advantage because we don’t know how to act, except generally.
But this lack of certainty in action only applies to the papal question. There is certainty that all the V2 popes are heretics. We know the doctrines they violated (and continue to do so). On this, there is no doubt.