The Jump Boost effect is negated when a player makes the horse jump.
What I expected to happen was...:
A player-induced jump should make the horse actually jump higher than usual due to the Jump Boost Effect.
What actually happened was...:
The Jump Boost Effect only occurs when a rider is off of the horse.
Steps to Reproduce:
1. Spawn a Horse
2. Use an NBT Editor to put Jump Boost on the Horse.
3. Have the horse jump up on its own and then look at the difference when you have the horse jump.
Notes: I have included a world where you can notice the difference between a player-induced horse jump and a regular horse jump with the Jump Boost effect. In my world I set up a bedrock room with walls around it; when the horse jumps with the Jump Boost effect, the horse will be able to have its feet over the wall. When the player makes the horse jump at max jumping power, the horse wouldn't have its head over the wall.
I know that the Jump Boost effect is not obtainable via a potion using regular means, so I am aware that this bug might be invalid and I will keep updating the versions even when this bug has been resolved.