Custom "Damage" tag fails to work / randomize anyway when items with durability are dropped

The code below is supposed to spawn a powered Creeper, then Zombies holding 2 pieces of Lapis-Lazuli, geared with 10 pumpkin helmets at once facing to their right, a highly damaged leather tunic and undamaged leather boots and leather pants :

setblock ~ ~ ~ minecraft:mob_spawner 0 replace {
EntityId:Creeper,
SpawnData:{powered:1},
SpawnPotentials:[
    {Type:Zombie,Weight:1,Properties:{
        Equipment:[
        {id:351,Count:2,Damage:4},
        {id:301,Count:1,Damage:0},
        {id:300,Count:1,Damage:0},
        {id:299,Count:1,Damage:76},
        {id:86,Count:10,Damage:1}
        ],
        DropChances:[1.0f,1.0f,1.0f,1.0f,1.0f],
        CanPickUpLoot:0,
	PersistenceRequired:1
    }}
],
Delay:0,
SpawnCount:1,
RequiredPlayerRange:16,
SpawnRange:6,
MaxNearbyEntities:1,
MinSpawnDelay:0,
MaxSpawnDelay:1
}

DropChances is set to force these Zombies to drop all their gear when killed.

However, when killed, even by drowning or any other mean that doesn't affect the durbility of the armor, here is what I got :
_some Rotten Flesh (as expected)
_10 pumpkins with unusual data value (as expected)
_2 pieces of Lapis-Lazuli (as expected)
_Moderately damaged Leather Boots, Leather Pants and Leather Tunic (I expected untouched boots and pants and nearly worn out tunic. Please note that the dropped Leather Tunic is actually less damaged than expected)

Note : the damage on this pieces of dropped armor seems to be often the same but not always. May be related to MC-36927