TNT on fire burns up like wood or leaves (it doesn't explode)

TNT doesn't explode when on fire

What I expected to happen was...:
Hissss, Kaboom (explosion)

What actually happened was...:
It burned up like a tree

Steps to Reproduce:
1. Place TNT
2. Place Lava, wait for a fire
3. Notice the lack of explosions (need I go on...)