An interesting question might be what about those lackadaisical Catholics who are told that missing Mass is a grave sin but do it anyway and don’t firmly believe that it is a grave sin. Would they always be guilty of a mortal sin? What exactly is full knowledge?
For example, say a doctor said that you must stop taking antacids while you are on a medication. You, however, don’t understand why taking antacids would be wrong. Now, you have knowledge that taking antacids with your medication is wrong but you lack the understanding of why taking them are wrong. With that knowledge how culpable would you be for the negative side effects if you decided to take antacids with your medication?
Now, certainly you’d be more culpable than a person taking antacids with that medication who has no knowledge that the two shouldn’t be mixed. However, would you be as culpable as a person taking the medication and antacids who knows they shouldn’t be mixed and why they shouldn’t be mixed? What the person in a situation in which they are told doing something is wrong by a trusted individual (like their doctor) but lack understanding of why it is wrong should abstain from doing what is wrong and endeavor to find out why it is wrong.