Example:
Place a repeating command block with this command:
/item replace entity @e[type=zombie] armor.head with air
Active it, then run this command:
/summon zombie ~ ~ ~ {ArmorItems:[{},{},{},{id:"minecraft:iron_helmet",Count:1b}]}
A zombie wearing iron helmet will appear, but if you check its data:
/data get entity <uuid of that zombie> ArmorItems
The command will return this:
Zombie has the following entity data: [{}, {}, {}, {}]
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Comments 2
Thank you for your report!
We're tracking this issue in MC-197737, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MC-197737 (as stated in comments there, armor too; report was just not changed to reflect that).