But many R&Rs do decide whether what Francis says is infallible or not...just as Xavier did with Siricius.
By now we all should know the pope is only infallible when he defines a doctrine ex cathedra. So there really is nothing for R&R to decide as far as Francis' infallibility is concerned because he has never defined a doctrine ex cathedra.
It is worth reminding that a doctrine is a truth or belief that all of the faithful within the Church has believed always and everywhere. That is what a doctrine is.
A dogma is nothing more than a doctrine, defined ex cathedra. Any and all new doctrines, like all those of V2, will never be defined ex cathedra because they are new doctrines and were never believed by all the faithful all of the time.
This is the decree of V1, it states when the pope, and only the pope is infallible, only when he speaks ex cathedra, that is
"when he defines a doctrine concerning faith or morals to be held by the whole Church" This is when doctrine becomes defined dogma, and this is the absolute extent of the popes' infallibility per V1.
Some theologians of the last few centuries opined V1's dogma on papal infallibility was greatly lacking, so they took it upon themselves to reward popes with additional infallibilities. Most of the faithful people accept these "add ons" or additional infallibilities as if they are de fide teachings of the Church, which helps explain why we are in this crisis. And those who believe them to be authentic teachings of the Church generally fall into one of three categories or some combination of them....1) Confused, 2) Novus Ordo 3) Sedevacantist.