Turtles do not suffocate if inside of solid block. see image.
Also happens to most other mobs.

Steps to reproduce:

1. Dig a 3x2x2 hole.
2. On the shorter side place two grindstones and any solid block on top of them.
3. Summon a turtle, and give it regeneration effect.
4. Push the turtle as close as you can to grindstone.
5. On the opposite site of grindstone, aim at the highest block and place sand.

Expected result:
The turtle should be getting suffocation damage.

Observed result:
The turtle doesn't get suffocation damage.

Notes:
The turtle does get suffocation damage if you replace grindstone with any solid block, then push the turtle to the side and place a block on the opposite wall the turtle is touched.