Bara för att de två vapenna är gjorda av samma material betyder inte att de är lika starka / kraftiga, särskilt i Marvel-universum. Olika tekniker i deras design, skapelse och förtrollningar verkar alla ha en roll i vapens ultimata kraft. Thor och den som förfalskade vapnet är båda övertygade om att detta är det mäktigaste vapnet som de någonsin har skapat, så det är anledningen att vi inte borde jämföra dem på en enkel materialnivå.
Vi måste också överväga att Thor hade nackdelar i båda striderna med Hela. I början hade han hammarens kraft men saknade en sann förståelse för sin egen kraft. Till sist förstod han sin enda makt, men saknade ett vapen för att kanalisera den kraften genom. Detta passar också i det i början av Avengers: Infinity War :
He still has that power, but no sufficient weapon, and thus loses easily to Thanos and his servants. But come the end of the movie, he has both his powers and a great weapon to focus it through, and comes out a much greater threat.
När det gäller den specifika träffen frågar du:
The implication does seem to be that Stormbreaker combined with Thor's powers are indeed formidable enough to stand up to the power of the infinity stones, at least to some degree. Considering individual stones are capable of being destroyed from a variety of "non-infinitely powerful" means, I think it's safer to consider the stones as sources of incredible power rather than omnipotent power. That is to say, they are incredibly powerful, but not "all powerful", and can be overcome to at least an extent given other sufficiently powerful forces.
Också:
The stones, like any other weapon, are only as powerful as how they're used. If Thanos' goal was just using them for their unfocused power to stop Stormbreaker, and it overcame that, then that's one thing. If he had used the stones to cause the axe to unexist, or reverse time on it, would that have had an effect? Maybe, maybe not. All we know is that however he chose to act against it, it was at least more powerful than that, and thus he was hit.