When ever I fall off a cliff, or height, I expect my diamond armour to decrease fall damage, however, it doesn't.

What actually happens is that I take fall damage, equal to the amount if im not wearing armour. This is affected on all armour types.

How to reproduce it:
Wear any type of armour.
Fall off from a height (5-10 blocks is preferable)
And the armour does not decrease fall damage