Shields do not block damage while the player faces straight up

The bug

The shield does not block attacks when the player faces straight up, even if the enemy is above the player.

How to reproduce

1. Ensure you are in Survival or Adventure mode
2. Hold a shield:

   /item replace entity @s weapon with shield
   

3. Face directly upwards
4. Summon an arrow directly above you:

   /summon arrow ~ ~20 ~
   

5. Attempt to block the arrow with the shield
   → The shield does not block the arrow

Code analysis and fix

Code analysis by Maity can be found in this comment.