Baby (zombie) villagers have different textures depending on profession.

Non-plains baby zombie villagers and baby villagers are missing their hats when applied certain professions using commands. This also affects naturally spawned baby zombie villagers because they spawn with professions.

The affected professions depend on biome types.

Example command to reproduce: /summon minecraft:zombie_villager ~ ~ ~ {VillagerData:{type:swamp,profession:shepherd},IsBaby:1}

To reproduce with a baby villager, get an adult villager with an affected profession, trade with him and run /data merge entity @n[type=minecraft:villager] {Age:-24000}.