What I expected to happen was:
When I used the /replaceitem command, the diamond block would render on the skeleton/myself

What actually happened was:

When I used the /replaceitem command , it did not render the diamond block on the skeleton/myself

Steps to Reproduce:
1. Run this command: /replaceitem entity @p slot.armor.head 0 diamond_block
2. Notice that the diamond block does not render visually on the player

Device NVIDIA Shield Tablet K1

Edit by 576874: added repo steps and updated title

Original: When I use the /replaceitem command to make mobs wear blocks on their heads, the command says it's successful, but the blocks that are supposed to be on their heads are invsible. I used the command to place a diamond block on a skeleton. The result was he stopped burning, but the diamond block on his head was invisible.