The Bug:

When a mob is ridden by a player and is attacked, it very rarely receives vertical knockback. This is not the case when non-player entities ride mobs. Below, I've attached some comparison videos of how vertical knockback is received to horses ridden by players, and to horses ridden by non-player entities.

MC-164558 - Knockback dealt to horses with player riders.mp4
MC-164558 - Knockback dealt to horses with non-player riders.mp4

Steps to Reproduce:

1. Summon a horse with a saddle equipped on it by using the command provided below.

   /summon minecraft:horse ~ ~ ~ {Tame:1b,SaddleItem:{id:"minecraft:saddle",Count:1b}}

2. Get two players and label them "Player A" and "Player B".
3. Get "Player A" to mount the horse and have "Player B" attack the horse multiple times.
4. Have both players observe the knockback dealt to the horse.
5. Take note as to whether or not mobs ridden by players very rarely receive vertical knockback when attacked.

Observed Behavior:

Vertical knockback is very rarely received.

Expected Behavior:

Vertical knockback would be received consistently.