Put the summary of the bug you're having here
What I expected to happen was...:
I expected to jump extremely high but jumped normally with the horse. I even splashed jump boost on the horse.
What actually happened was...:
Jumped normally
Steps to Reproduce:
1. Tame a horse
2. Get a jump potion 1 or 2
3. Try to jump at max jump on the horse. (Bug found)
This issue has been reopened in MC-121788. Please go there if you want to add anything.