The shied won't deploy if you directly look at a mob and you are in his clicking range. You have to look in the air to deploy the shield while fighting mobs.
To reproduce the bug just try to deploy your shield while looking at a mob near you. No matter what hand or even if you have another item in the opposite hand or not.
Exemple video : https://youtu.be/9orc3x1fTC4
- duplicates
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MC-160901 Using an item while looking at an entity is no longer possible
- Resolved