Squid and all other aquatic mobs can spawn at any height in the world, from y=1 all the way up to y=255, as long as its an ocean biome. This includes squids, dolphins, turtles, drowns, tropical fish, puffer fish, cod, and salmon.

This might be fine for most aquatic mobs, but squid are only suppose to spawn from sea level (y=62) down to y=40.

How to reproduce:
1. Go above any ocean biome at world height, and build a chunk sized pool of water, as deep as you like. Make sure the bottom of the pool is made of solid, spawn-able blocks.. Nothing will spawn if the bottom is spawn proof >.>
2. Set up a repeating command to kill everything not around you:
`/kill @e[type=!player,rm=30]` this will allow things to spawn in your pool.
3. Observe as fish, dolphins and maybe even drowns spawn in the pool, at world height.

You can repeat this at the bottom of the world as well, floor of your pool at y=0, and water above it(make sure there are no solid blocks above your water, or anything. That will also prevent all spawns)

For more info on this bug, and other issues with squids/aquatic creatures check out my recent bugrock of the week episode on this topic:
https://youtu.be/Qt7npxE9UzM