‘It is [sic] can no longer simply be said that all those in any “irregular” situation are living in a state of mortal sin and are deprived of sanctifying grace. More is involved here than mere ignorance of the rule. A subject may know full well the rule, yet have great difficulty in understanding “its inherent values”, or be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin.’ (AL 301)
The theologians claim that this statement understood as meaning that a Catholic believer "can have full knowledge of a divine law and voluntarily choose to break it in a serious matter, but not be in a state of mortal sin as a result of this action", is heretical.
Amoris Laetitia seems to suggest, at least with the second part of the hypothetical "A subject may know full well the true, yet… be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin" that an individual may, due to being obligated in conscience to act a certain way ("does not allow him or her to act differently… without further sin"), not bear grave guilt in the violation of the "rule" that puts them in an "irregular" situation.
In this hypothetical, Amoris Laetitia is envisioning somewhat who is caught between two (perceived) obligations: the obligation of the rule in question, and the obligation of another rule that applies in this particular circumstance. For example, some one who married for a few years, without children, was divorced by the first spouse, has civilly remarried in ignorance of the fact that divorce cannot dissolve a valid marriage, been living in this civil marriage for many years and had children with the second partner, may come to learn of the indissolubility of marriage and the consequent obligation of fidelity to the first spouse, yet at the same time believe themselves bound by the grave obligation of justice to their second partner (including sexual giving of self to the partner). Such a person, caught between two apparent divine obligations, may discern in conscience that their greater obligation is to the current partner; the argument of Amoris Laetitia implies that such a person does not thereby necessarily commit a mortal sin.
One might quibble about whether such a person really knows "full well" the rule, but that is more an argument about words than a substantial argument about the matter in question.
The two major points one might raise are: is it possible, without grave fault, to believe that one divine law obliges one to break another divine law? In general St. Thomas Aquinas held that ignorance regarding divine law was culpable, and so, in speaking about the case of a perplexed conscience, which obliges a person to do something contrary to divine law, he argues that a person, so long as they are in that ignorance, sins by disobeying their conscience, since they do not follow the divine rule as it is presented to them by their conscience, and likewise sines by following their conscience, since they do not follow the divine rule that they should have know. He does make exceptions for insane persons, who are not capable of any moral action, but it is not clear whether he would make any further exceptions. However, considering the various external factors that lead someone to accept something as a moral obligation, it seems necessary to admit that in some cases, ignorance regarding divine laws may be not gravely culpable. If this is true, someone who erroneously believes that he is bound by a "higher" (as far as he perceives it) divine law to disobey another divine law, would be inculpable. One may not be obliged, by catholic Faith, to believe that there are such cases. But it is not in the least heretical to hold that there are; and indeed, this very statement of Amoris Laetitia provides a reason, based on church authority, to hold that there are, even if a small reason considered in itself.
The first part of the hypothetical "may… have great difficulty in understanding 'its inherent values'", I find harder to interpret, and potentially more problematic. But whatever exactly is meant by understanding "its inherent values", it seems to imply at any rate not really having "full knowledge" of a divine law in a morally relevant sense. I may come back to this, if I think I have something enlightening to say about a relationship between "knowing a rule" and "understanding its inherent values".