Grass blocks/path blocks falsely converted to dirt

I was building in Minecraft when something strange happened. I saw a path under a sign, and grass under vertical trapdoors were converted to dirt. I think this is a bug because its never happened before and doesnt make much sense.
Steps to Reproduce:
1. Place Grass Block Or Path Block
2. Place A Vertical Trapdoor Or A Wall Sign On Top
Observed Results:
The Grass/Path Is Converted Into Dirt
Expected Results:
It Should Not Be Converted Because It Hasn't Before