The Bug:

When an entity is ridden by a player and is attacked, it very rarely receives vertical knockback. This is not the case when non-player entities ride other entities. 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-245420 - Knockback dealt to horses with player riders.mp4
MC-245420 - Knockback dealt to horses with non-player riders.mp4

Steps to Reproduce:

• Summon a horse with a saddle equipped on it.

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

• Get two players and label them "Player A" and "Player B".
• Get "Player A" to mount the horse and have "Player B" attack the horse multiple times.
• Have both players observe the knockback dealt to the horse.
• Take note as to whether or not entities 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.