Minecraft wiki entry for Jump_Boost, associated text: When at maximum level, jumping is not changed, but a player takes 256 × 128 less fall damage.
At level 15, the increased height exceeds the maximum possible damage reduction, resulting in fall damage upon landing.
At level 33, a player jumping on the spot is killed by fall damage upon landing, receiving the death message "<player> fell from a high place".
In Java Edition, at level 128, the player can no longer jump because of integer overflow.
At high enough levels (achievable only with commands), the player may jump hundreds of blocks high, enough to exceed Y=256.