Jax disappeared

Finally he got his real weapon, the invisibility cloak.

Though as funny as it sounds, your Jax didn't actually disappear. He just got stuck in the air, you could see him flying upward. I think it probably happened because your Jax was pushed by the Dragon with his AoE knockback while leaping that somehow cause this. Or just something wrong with the leap that cause you to go Michael Jordan.

As for why Jax was still taking damage and hitting things fine, because this game does not have a z-axis. It's a 2D game as far as its calculations are concerned.