Perhaps he's only invulnerable to an impact he sees coming.
He was able to catch the bullet when he was facing the gun, and the baseballs when he was facing the launcher, but the shot from behind didn't allow him to 'prepare his body' for the impact.
He was able to catch the bullet when he was facing the gun, and the baseballs when he was facing the launcher, but the shot from behind didn't allow him to 'prepare his body' for the impact.