Is that his E gives invulnerability based on where the enemy champion is, not where the damage is coming from. Usually these are the same thing, but that's not always the case with certain bouncing attacks (Jhin's Q, Sivir's W, etc.), possibly summons (Malz W and the like, although I haven't tested that myself), and, most notably, boomerang attacks (Ahri Q, Sivir Q, Swain E, etc.). The damage's original source (i.e. the enemy champion) is in front of him, so the damage is blocked, however, the damage itself is coming from *behind* him, which means that it *logically* shouldn't be blocked since it's hitting from behind the shield.
I'd rather it be the other way around. Whether or not it blocks should depend on what side he gets hit on, not where the champion attacking him is, bringing it in line with abilities such as Braum's E, and bringing it in line with the logical conclusion of *hitting someone from where they don't have their shield means that they won't block the attack with that shield*.