Zombies have rendering problems with block on head

I think some parts of this were already reported but I couldn't find them, so I am sorry if this is a duplicate.

Also for villager zombies it seems like the block and the head won't rotate simultaneously (see screenshot).

So I really wonder (I know this could sound naive) why MOJANG doesn't just copy and paste some parts that work already for other mobs:
Example: The rendering of blocks for baby villager works, while the rendering for baby zombies doesn't; also the rendering of armor for baby zombies works, but for villager babies it doesn't