Unbreakable helmets still take damage when worn by mobs in the sun

I noticed that helmets with the NBT-Tag Unbreakable: 1 still take damage and break when worn by mobs in the sun.

What I expected to happen was...:
The helmet stays and does not break or take damage.

What actually happened was...:
It broke and took damage, revealing the monster to the sun.

Steps to Reproduce:
1. Create a command block with the command:

summon Zombie ~ ~1 ~ {Equipment: [{},{},{},{},{id: minecraft:iron_helmet, Count: 1, Unbreakable: 1}]}

2. Activate the command block, spawning the Zombie with an iron helmet
3. Wait a while and the helmet disappears