Armour doesn't flash red when taking damage

When armour is equipped on a mob or player, it will never have the red flash when taking damage that the actual mob does. This makes it hard to tell when something with full armour is taking damage, as most of the red tinted area is covered up by the non-tinted armour.

What I expected to happen was:
When a mob or player takes damage, the main texture along with any armour texture would flash red to indicate damage.

What actually happened was:
Only the main texture flashed red, with the armour texture staying normal coloured.

Steps to reproduce:
1. Find a mob or player with armour.
2. Make mob or player take damage in some way.
3. See lack of red flashing.