The bug

Baby zombies' heads are enlarged to create a larger 'infant proportioned' head, however this enlargement is only applied to the 'skin' portion of the model not the 'hat' portion of the model. The 'hat' is still being rendered but is inside the lager baby head.

Steps to reproduce

1. Apply the attached resource pack Test Pack.zip
   This resource pack adds a full-black hat to the drowned overlay (except front back and bottom face), zombies, zombie villagers and husks, and a full-white hat to the inner drowned texture

2. /summon zombie ~ ~ ~ {IsBaby:1b,NoAI:1b}
   /summon drowned ~ ~ ~ {IsBaby:1b,NoAI:1b}
   /summon husk ~ ~ ~ {IsBaby:1b,NoAI:1b}
   /summon minecraft:zombie_villager ~ ~ ~ {IsBaby:1b,NoAI:1b}

3. Observe the hat layer not being visible.
4. Inspect the inside of the head via spectator mode.
5. Observe the hat layer is inside the head.
6. Additionally, for zombie villagers, observe the hat slightly sticks out below the head.

Notes

1. This also affects drowned, but not the overlay texture; the second layer exists on both textures, and both hats are affected.
2. When this is fixed, the outer layer of the drowned can actually get placed as the second skin layer of the base drowned texture, and the outside layer removed entirely. The same result would be achieved with a texture file less.