As for the MC, considering that an MC who passes all the stat checks and uses the right skills, i.e. in the best possible situation, is the canonical, I think I can only remember of two situations in which the MC's defeat is undeniable: 1-When the Wu children teamed up to beat him; 2-When the MC ran away from the shadow guys' ambush."Invincible", but with at least four defeats on his shoulders.
I can think of two other situations, but it depends on the point of view. One is against LC, but since only Ruo admitted defeat at that point, I can't say it's an unquestionable defeat. Another is against the Demon Executing Squad, he won, but it's thanks to the Devil who took control, otherwise he would have died, so it also depends on the point of view.
I'm not denying other defeats, but I could only remember these.