Setting up a system to immediately use /data to remove ArmorItems NBT data from entities will not update the visuals in-game. The entity does lose the armour, but still appears to wear it.

To replicate, place a repeating/unconditional/always active command block with the following command: 
     execute as @e[tag=!Updated] run data merge entity @s {ArmorItems:[{},{},{},{}]}

Then, a chain/conditional/always active command block with the following command, chaining from the first:

     tag @e[tag=!Updated] add Updated

Finally, summon a zombie to test with:

     /summon minecraft:zombie ~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:diamond_helmet",Count:1b}]}

The zombie will appear to have a diamond helmet equipped, but will still burn in daylight and using /data get will reveal that it has no ArmorItems.

 There is an identical issue with the HandItems tag.