Vexes float upwards underwater

The bug

Vexes float upwards underwater, making them unable to attack players underwater. When they enter the thin air gap formed when a solid block is on top of a water source block, they helplessly bob up and down and do not fly away.

How to reproduce

1. Go to any ocean in your world or create a big room filled with water
2. Switch to survival mode and give yourself some water breathing and night vision

3. /summon minecraft:vex ~ ~ ~ {Invulnerable:1b}

4. Notice how the vex behaves

The vex will immediately float upwards and then attempt to fly down to reach the player. Because of the buoyancy, I was completely safe from the vex's attack when I took the screenshot 2018-02-09_12.09.38.png.

2018-02-09_11.44.32.png was taken in an enclosed water room and I was in creative. The vex in this screenshot doesn't move apart from bobbing up and down in this air pocket.