Items are invisible when held by mobs.

When you use the /replaceitem command on mobs such as all types of zombies, all types of skeletons, vindicators, evokers, and pillagers, it doesn't render as it is invisible (Does not effect drowned and armor stands)

Steps to Reproduce
1. Use /replaceitem entity. @e[type=Zombie] slot.weapon.mainhand 1 trident
2. Look at the hand on the mob.

Note: All items don't actually render on mobs who have a main-hand or off-hand