Mobs holding spears cannot redirect fireballs or apply knockback on certain entities unlike players

When the player is either attacking or charging the spear at a fireball, the player will redirect the fireball. The player can also knock backback entities like boats and Primed TNT.

However a mob (like Zombie) cannot redirect a fireball the same way. It is also unable to apply knockback on Primed TNT or boats (but does damage the boat).

Steps to Reproduce:
1. Summon a zombie holding a spear on level ground

/summon zombie ~3 ~ ~ {equipment:{mainhand:{id:"minecraft:iron_spear",count:1}}}

2. Summon a fireball at eye height

/summon fireball ~ ~1 ~

3. In Survival mode, manoeuvre the Zombie such that it must collide into the fireball on its path to attack you

1. You should hear a spear attack sound as it collides with the fireball, but no redirecting

2. Now try this yourself and this will redirect the fireball

You can repeat the same steps with other entities like the TNT and boat but place them on top of a block over level ground so it remains eye level with the Zombie.