Villagers are not working properly through a weakness on a zombie villager bit me with a golden apple nothing happens.
Skyblock? How long did you wait for the zombie to change? What happened to the zombie? Despawn, die? Zombie villagers can still hit you while they have weakness.
Also did you right click the z-villager WHILE the Splash potion of weakness effect was active? Also refer to that to make sure you did it correctly.