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
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anthony cicinelli
gaspoweredpick
Duplicate of MC-160901
Oh, I didn't find it. Then mine can be deleted.
Can Confirm when in clicking range of a mob you can not deploy your shield with off or normal hand