Helmets/hats (items in the helmet slot) are aligned differently on villagers and zombie villagers than on illagers and wandering traders
The bug
Illagers, pillagers and wandering traders items on their heads are aligned differently than villagers and zombie villagers.
Reproduction Commands
/summon villager ~ ~ ~ {NoAI:1}
/summon wandering_trader ~ ~ ~ {NoAI:1}
/item replace entity @e[type=minecraft:villager,sort=nearest,limit=1] armor.head with minecraft:carved_pumpkin
/item replace entity @e[type=minecraft:wandering_trader,sort=nearest,limit=1] armor.head with minecraft:carved_pumpkin