Zombie villager butchers will retain their biome variant headwear whereas normal villager butchers will lose their biome specific headwear and only display the butcher headwear.
Steps to reproduce
Summon the differing villagers and observe their headwear.
/summon villager ~ ~ ~ {VillagerData:{level:2,profession:"minecraft:butcher",type:"minecraft:snow"}}
/summon zombie_villager ~ ~ ~ {VillagerData:{level:2,profession:"minecraft:butcher",type:"minecraft:snow"}}
this bug also exists for desert butchers, their zombie form wears a hat but not as villager.