The formal declaration format used to canonize would seem to say yes, a statement of finding of sanctity by Pope would be infallible.
Believe that is substatiated by the historical fact that early “saints” not declared by that formula and only be bishops xare the iffy ones now.
All Popes, or rather, all the most recent Popes, starting from the Second Vatican Council, are presented as saints. It is not by chance that the canonizations of John XXIII and John Paul II have left in their wake the canonization of Pius IX and the beatification of Pius XII, while the cause of Paul VI moves forward. It almost seems that a halo of sanctity must envelop the Conciliar and Post-conciliar eras, to “infallibilize”an historic age which saw the primacy of pastoral praxis assert itself over doctrine in the Church.