Steps to reproduce:

1. Spawn a bunch of hostile mobs
2. Equip a shield;
3. Set gamemode to Survival;
4. Activate the shield, let the monsters hit you and don't stop pressing the button;
5. When you've deflected enough attacks, stop pressing the button.

Expected results:
Each hit subtracts a bit of durability and updates the bar.

Observed results:
Although every hit is subtracting a bit of durability, the bar doesn't get updated until you stop pressing the button.