Mobs holding Knockback swords in vehicles deal knockback in the wrong direction

Steps to Reproduce:

1. Spawn a zombie carrying a sword enchanted with Knockback and riding a boat:

   /summon boat ~ ~ ~ {Passengers:[{id:zombie,HandItems:[{id:iron_sword,Count:1,tag:{Enchantments:[{id:knockback,lvl:2}],AttributeModifiers:[{AttributeName:generic.attack_damage,Amount:-0.9999,Operation:2,UUID:[I;1,2,3,4]}]}}]}]}

   /summon boat ~ ~ ~ {Passengers:[{id:zombie,HandItems:[{id:iron_sword,Count:1,tag:{Enchantments:[{id:knockback,lvl:2}],AttributeModifiers:[{AttributeName:generic.attack_damage,Amount:-0.9999,Operation:2,UUID:[I;1,2,3,4]}]}}]}]}

2. Walk around the zombie and notice that there is only knockback dealt in one direction - the direction his head is facing

Expected Behaviour:

The player should always be knocked radially away from the zombie.