The 1998 CDF Commentary on Ad tuendam fidem (paragraph 11) gave examples of "truths connected to revelation by historical necessity and which are to be held definitively". One was "the legitimacy of the election of the Sovereign Pontiff".
I have near my desk a useful little CTS pamphlet (1958) (it cost me 6d when I was a smart little chap of seventeen ... money well spent): The Popes from St Peter to Pius XII. Jolly good for dates and simple summaries. Based upon the Annuario Pontificio of 1904 and 1905. Each pope has his own number. Numbers 112, 114, 116: Pope Formosus was posthumously deposed by his successor Stephen VII but rehabilitated by his successor Theodore II. 119 Leo V: "Regarded as probably an anti-pope". Leo VIII (993): "His election is of doubtful validity". 133 Benedict V: "His election is of doubtful validity". 146A Sylvester III: "His election is of doubtful validity". 147 Gregory VI: "His election therefore is of doubtful validity". 200 Urban VI: "The election ... has however been generally deemed valid".
Quite a bag of popes here about whose legitimacy (or illegitimacy) greater or lesser doubts are openly expressed. If the list-makers are themselves in such states of doubt, how can the ordinary Catholic know how to submit to the requirements ("to be held definitively") implied by the Commentary? Urban VI, number 200, is fairly important. Upon his status depends the question whether, from 1378 onwards, the 'Roman' popes or the 'Avignon' popes were the true line. The schism lasted until 1417 and there has never been, as far as I am aware, a definitive resolution of the uncertainties before that date. To say that his "election has been generally deemed valid" seems to my untutored and uncanonical eye much weaker than to say "the legitimacy of his election is to be held definitively". And, to add to the confusions, respectable authors have imported an axiom Papa dubius Papa nullus (I can't discover who first deployed it) to argue that, during the Great Schism, there was for four decades no Pope at all. You can see why this has some appeal: after the Council of Pisa (1409) the 'Roman' pope, Gregory XII, commonly regarded as the 'real' pope, did not have jurisdiction beyond Italy; the 'Avignon' 'antipope' Benedict XIII was accepted by nobody outside a little Spanish town called Peniscola; and the Pisan 'antipope' Alexander V held sway over the rest of the world.
But ... Oh dear ... I've just discovered that my schoolboy list is out of date. It is rather different from that in the current Annuario Pontificio. Apparently, the list in my leaflet dated back to 1904/5, but there was a revision in the 1940s. Indeed? Then the scholars who did that revision, the person responsible for that revision, did not, apparently, regard the previous list as definitive tenendum. And ... Ah!! God bless Wikipedia! There was more revision in 2001! Gracious! So those responsible for doing that did not regard the list which they picked up to revise as being definitive tenendum. So, why on earth ...
Yeah ... I know ... I am trying your patience. The Commentary was simply clobbering modern sedevacantists ... and so it jolly well should ... all power to its elbow. There was no intention to spray irrelevant anathemas all over historical pedants who hold divergent views about the status of long dead popes and antipopes. Nor was it excommunicating the chappies who keep revising the list in the Annuario every generation or so. Have some common sense, Fr H. Fair enough.
But ... and this is my point ... I think the methodology of the Commentary is lacking in logical rigour. It cannot really mean exactly what it says. (It was not approved by the then pope ... nor approved in forma specifica.) And it risks dragging into disrepute the entire concept of "to be held definitively". In strictly logical terms, why should it be 'what-the-Hell-who-cares' to dispute the validity of the election of Gregory VI but totally terrible to question that of Francis I? Over the years, apparently, definitive tenendum gradually and gracefully evaporates. As the magicians say, Now you see it, now you don't.
My tentative conclusion (I really am open to well-argued elucidations but not to irritable rants) is that this paragraph in the Commentary is intended to point pastorally and reliably to where authentic Church Life is, here and now, to be found and lived. It is to be found in communion with Francis, who really is Pope (not in sedevacantist groups). Sure pointers to sure realities; reliable notice-boards about real minefields where Death truly lurks. That is why, here and now, observing it as definitive tenendum keeps you safe, and is important. But, despite its rather fierce appearance, this language is not intended, cannot be intended, as an implacable iron rule to resolve every doubt in past history - it just doesn't work - nor can it, need it, be an eternal mill-stone round the Church's neck.
It cannot be wrong to speculate on what future generations of editors of the Annuario Pontificio might decide about the status of Francis I. Although it may be imprudent or a waste of time better spent otherwise!
So ... yes ... you do have to be in communion with Francis. It's essential to you and your salvation in this precise moment that God has placed you in. But that in no way prevents a harsh judgement being made in the future with regard to the present occupant of the Roman See ... just it has been upon Leo V or Leo VIII and all the rest of those I list above. That judgement may be made by future popes and Councils; more probably, it may be made simply by the evolution of historical consensus.