Grass Item Durability Bug

I have been playing with TNT lately and I have found a bug on the grass.

What I expected to happen was...:
I expected when the TNT explode, the grass, and the dirt layer will be destroyed

What actually happened was...:
When the TNT "kaboom", the grass block isn't destroyed but the dirt explodes.

I found that grass has a durability as like Obsidian which is broken when multiple explosion happen with the grass block