Let us say that I commit a blunder in a chess game. One that is deserving for a ??, a mate in one let us say. My opponent misses it. With what mark should the move be written in a pgn? Left markless, or with a ?!. Or perhaps with something else?
In the specific case you give, both the move that allowed the mate in one and the move that failed to take it are clear ??'s: they're both absoutely terrible moves.
A blunder is a blunder regardless of the opponent's response. Allowing mate in one in a playable position always deserves a ?? if you are adding annotations to a game.