Glow Squid not spawning

Glow Squids do not appear to be spawning, despite meeting the required conditions.

 

How to replicate

1. Open a new flat world.
2. Build a stone box below y = 63. I made mine 7x15x7.
3. Fill with water. I left a two block air gap between the top of the water and the roof of the box.
4. Leave for a while. Notice how no glow squids spawn.

 

Am I missing a requirement or is this a bug?