Zombies just reuse the humanoid arms animation for holding something in the offhand, as they don't seem to have an animation. But when you give them a shield in the offhand it goes entirely off of the normal arm position and through the head.

How to reproduce:

1. Spawn a zombie and name it h
2. Do /replaceitem entity @e[name=h] slot.weapon.offhand 0 shield

Result:

The shield will be sideways inside the head.

Expected result:

The shield should be fixed around the arm like the totem.