The creaking's spawning does not check if there are blocks in the middle or head level of their bodies.

To reproduce,

1. Open a superflat world with void preset.
2. Place a layer of any block with solid top surface (I used glass in the pictures), then leave 1 block of air gap in between and place 1 or 2 more layers of solid block on top.
3. Place bottom slab on the top solid block to prevent creaking spawning on it
4. Activate a creaking heart nearby. Remember to cover its top surface with bottom slabs too.
5. Observe that creaking can spawn in the air gap.