Mobs don't swap their armour for better armour

Mobs (with CanPickUpLoot set to 1b) don't swap out their armour for better ones.

Steps to Reproduce:
1) Run

/summon zombie ~ ~ ~ {CanPickUpLoot:1b}

2) Drop an iron helmet and wait for it to wear it
3) Drop a netherite helmet and note that the armour isn't swapped