If Fencer Y performs a hit valid on target but it is later discovered to be invalid due to a rule violation by themselves, what should the referee conclude?

In this situation, the referee needs to consider the rules surrounding valid hits and the implications of any rule violations by the fencer who scored the touch. If Fencer Y initially performs a hit that is valid but later is found to have committed a rule violation, this violation supersedes the validity of the hit.

When a fencer illegally obtains a touch, the integrity of the scoring system must be maintained. The appropriate course of action is to award the touch to the opposing fencer, which ensures that the competitive integrity is upheld. This is rooted in the principle that a violation cannot enhance a fencer's score. The referee must prioritize the adherence to the rules over the outcome of the hit, ensuring fairness in the competition.

Consequently, awarding the touch to Fencer X is the correct decision, as it rectifies the unfair situation created by Fencer Y's infringement.