I may be wrong, but my understanding of Stormshield is that it may or may not block opposing attacks depending on if the top card of your deck is a pillar (or pendulum). It is essentially a Fog Mantle with [pillars in your deck]% evasion chance.
But what happens instead is that each incoming attack has a 100% chance to mill a pillar if there is one left in your deck.
For example if the opponent attacks with 14 creatures and you have 10 pillars or pendulums left in the deck, all 10 pillars/pendulum will be milled away.
This happens consistently, this is not a one-tum deck randomisation oddity.