I've created modified spawn.json to make zombies spawn with armor (I'm using NBT rather than armorhelmet, etc. because I'm trying to make it so the zombies always drop their equipment).
Here's what I'm doing:

[
  {
    "mob": ["minecraft:zombie","minecraft:husk"],
	"random": 0.05,
	"onjoin": true,
	"nbt": {ArmorItems:[{Count:1,id:iron_boots},{Count:1,id:iron_leggings},{Count:1,id:iron_chestplate},{Count:1,id:iron_helmet}],ArmorDropChances:[1.0f,1.0f,1.0f,1.0f]}
  }
]

I've truncated it down to just one rule, the actual file has many similar rules with different equiment sets

The issue I'm having is that the ArmorDropChances part isn't taking effect; the zombies only drop some-to-no equipment. When I test this NBT with the command /summon zombie ~ ~1 ~ {ArmorItems:[{Count:1,id:iron_boots},{Count:1,id:iron_leggings},{Count:1,id:iron_chestplate},{Count:1,id:iron_helmet}],ArmorDropChances:[1.0f,1.0f,1.0f,1.0f]} the ArmorDropChances part does work; the zombie drops all its equipment every time.

Please note that I DO NOT want to modify the loot table, I want to actual modify ArmorDropChances so that the zombie only drops equipment it's equiped with.