The bug

Wind Charges cannot wind up or knock back some rideable mobs and their passengers.
This causes the player to be immune to Wind Charge knockback when mounted on an entity.

Steps to reproduce

1. Summon a horse.
2. Tamed it.
3. Saddle it (make it controllable).
4. Ride the horse.
5. Summon a breeze.
6. Notice that the wind charges can't effectively knock back the horse and its passengers.

All affected mobs:

1. Horse
2. Donkey
3. Mule
4. Skeleton horse
5. Zombie horse
6. Boat (raft)
7. Boat (raft) with chest
8. Minecart