Using /replaceitem to give Zombie/Zombie Villager a shield creates a large shield in the sky

Steps to reproduce:

1. Spawn Zombie/Zombie Villager and stand close to them
2. Execute the command /replaceitem entity @e slot.weapon.offhand 0 shield

Expected results:

The mob should be given a shield.

Observed results:

Instead of the mob having a shield in its hand, the shield spawns in the sky and follows the player as they move.