Let me begin by stating that this scenario is hypothetical.
The scenario is this. Let’s say that a Catholic goes to confession on a Friday night to a different confessor than his normal confessor, as he is out of town and wants to make a confession. He makes a valid confession and is absolved of all his sins. He goes home that night and does not commit any mortal sins, or any other venial sins. The next morning, he believes, based on his understanding and knowledge of mortal sins and the CCC, that he committed a mortal sin that morning. He immediately calls his regular confessor and that Saturday morning and is agreed to be seen right away for a confession. The confessor hears his confession and tells him that he did not commit a mortal sin and asks if the person has anything else he would like to confess, and he says no, I confessed last night, and all of my sins have been absolved and after examining my conscience I cannot recall any other sins to confess. The confessor at that time does not absolve him of any sins because he has nothing to confess. The person goes back home.
Now, let’s stipulate that the person actually did commit a mortal sin, but for whatever reason, the priest did not believe it to be so, and the confessor did his best to explain what had happened. Therefore, it was a valid confession, and nothing on the part of the confessor was done wrong.
Would this person still have the stain of mortal sin on his soul?
Would it be absolved in his next confession if he did not mention it thinking that it was not a mortal sin based on the priest’s interpretation?
PS: Sometimes my mind just creates these scenarios and they make me wonder, because I am sure that there is a concrete answer to these questions.