Not sure if it's a bug or intended, but as Braum's shield is intended to block projectiles, seems like a bug to me. I was playing the Braum when I managed this, Garen ult was the first thing to hit the shield so took no damage.
If he was targeting you, then the first spell to hit the shield is completely negated. If it was someone else, then it's totally a bug .-.
From Lol Wikia, I hope it helps.
ACTIVE: Braum raises his shield in the target direction, creating a barrier that intercepts incoming non-turret enemy projectiles and reduces the damage he takes through the shield. The first instance of damage blocked by his shield is negated completely.
Braums shield reduced damage from ALL ATTACKS that come from the direction he holds the shield up in, even if they are not projectiles.
It has the added benefit of intercepting projectiles that attempt to pass through it forcing them to hit him instead of their intended targets.
