NPC won't show items held in his hand

I used the command "replaceitem @e[type=npc] slot.weapon.mainhand 0 iron_sword" to a NPC, but found it was nothing appeared in his hands. 

Actually it happens in any mobs having component "minecraft:npc".

Even if I changed the render controller and other texture files, it won't work.

Steps to Reproduce:
1. Use the command replaceitem to change the item NPC is holding
2. Observe what's in his hand

Observed Results:
Nothing turned up.

Expected Results:
Holding a given item .