How to reproduce

1. Summon an zombie with diamond armour with

   /summon minecraft:zombie ~ ~ ~ {ArmorItems:[{id:"minecraft:diamond_boots",Count:1},{id:"minecraft:diamond_leggings",Count:1},{id:"minecraft:diamond_chestplate",Count:1},{id:"minecraft:diamond_helmet",Count:1}]}
   

2. Try to remove the armour with

   /data remove entity @e[type=minecraft:zombie,sort=nearest,limit=1] ArmorItems
   

   and see that it does not work.

3. However, removing only the helmet with

   /data remove entity @e[type=minecraft:zombie,sort=nearest,limit=1] ArmorItems[3]
   

   does work.