I have made my custom texture for every type of diamond item, the armor was in my list. in 1.8.X that doesn't work at all.
in 1.9.X/Pre-X you can see the texture doesn't appear transparent when it's enchanted.
1#If i only put my helmet that work...... (first image 1# ect)
2#Helmet + Boots: Only boots
3#Helmet + Boots + Leggings | Helmet/Boots + Leggins : Only leggings
4#Helmet + Boots + Leggings + ChestPlate | Helmet/Boots/Leggings + Chestplate : Only ChestPlate
-*When that is not enchanted all piece work properly.
5# When all my armor is clean. (you can see the white line of my skin)
6#When all my armor is renamed to gain the " NBT Tag '2' " *The diamond i'm holding is to show it's correctly transparent.

Update, since snapshot version 19w41a, texture pack transparency on armor is totally gone.
Update: In my opinion related to current 20w10a missing enchant Glint, both "Armor" rendering issue.