When the shield is on the player's off-hand, and the player blocks an attack from another player, a mob, or a TNT explosion there is no blocking animation. See the comparison between the main hand shield and off hand shield blocking animation.

Steps to Reproduce:

1. Spawn a hostile mob
2. Equip a shield in your off-hand
3. Switch to Survival mode
4. Block an attack from the mob

Observed Results:
There is no blocking animation.

Expected Results:
There should be a blocking animation.

Videos:
main_hand_shield.mp4
off_hand_shield.mp4