When I fly upwards with my elytra to hit the wither above ground, he always manages to fly above me. It doesn't matter how far up I go, the wither always goes further upwards, and now there's a wither on my world I can't deal with.

Steps to reproduce:
1. Summon the Wither, and wait for him to fly upwards.
2. Fly up to him with an elytra and attempt to attack him.
3. Repeat indefinitely.