Is there an official ruling on this point?
Presuming we are taking about 'Tumbling Escape'. The power card wording is clear. "Use this power when a Monster hits you with an attack." So the power can only be used if the Monster scores a hit, not when it misses. It is the die roll that determines hit or miss, and only the die roll.
Once the Monster has a hit we can choose to play the Utility Power card, and that card should take precedence, and cause the attack to miss entirely. I see no obligation to go back and change the result of the original die roll.
Given that a valuable power card is used, this interpretation seems quite reasonable.
Yep, tumbling escape is the power in question in my example. Everything you said here is exactly what I was thinking. I went ahead and played it that he took no damage whatsoever. Otherwise it makes the power, and others like it, pretty weak in my opinion.