The central issue of inference validity, that guarantees the correctness of reasoning and thus that of derived knowledge, depends on the definition of both implication operator and tautology. This paper studies the modus ponens validity in the case of Intuitionistic Fuzzy logic, in an experimental framework: considering 18 classical implication operators, it shows that validity usually does not hold for the classical definition of Intuitionistic Fuzzy tautology. It proposes two alternative, more constrained, tautology definitions, studying them with the same protocol, showing they make it possible to decrease the number of invalid implication operators.