Horses, donkeys and mules are limited to storing only one jump.

In a recent update like 1.20, horses, donkeys and mules gained the ability to remember when the rider commands a jump. Therefore, if you press the jump key while the horse, donkey or mule has no footing, it will remember the command and execute the jump once it touches the ground. However, the horse, donkey or mule can only store one jump at a time. Consequently, if you attempt this action twice, it will only execute one jump and will not jump again upon landing.