Creaking spawns on glass blocks

What I expected to happen was...:
Creaking shouldn't spawn on glass

What actually happened was...:
I have a layer of pale moss, a layer of eye-blossom flowers, pale moss carpet, and saplings, then a layer of glass. This forms a large circle and there are nine wood blocks in a 3x3 square in the middle. I expected the creaking to spawn on the wood square, but it spawns on glass. 

Steps to Reproduce:
1. Build a large circle of glass, with a wood block center. 
2. Place creaking heart and two pale oak logs in the proper orientation to activate the heart, near but not in the middle. 
3. Wait for night time

4. Watch creaking spawn on the glass