I currently use shields to protect myself from the ender dragons damage. I have so far done this bug twice with two different shields: first, be crouching with the shield activated. Second, get attacked by the ender dragon (after she swoops down from perching on the middle bedrock spot) you will deflect the first couple hits with the shield but her hit box goes behind you, and will kill you. When you respawn you will be crouching with your shield out, having it still in your shield inventory spot. hope these tips help you recreate it, its fairly easy to do