The force with which Thanos hit Iron Man in Infinity War most probably completely shattered his helmet.
In the fight, Iron Man’s suit was severely damaged and the left side of his helmet was destroyed.
But what we didn’t notice was how Endgame retconned a scene from Infinity War to foreshadow Iron Man’s death from the very beginning of the film.
In the 2018 film, Iron Man, Spider-man, Doctor Strange, and the Guardians of the Galaxy fought Thanos on the ravaged planet of Titan for the Infinity Stones.