Mangrove Roots are non-flammable

I tried burning a mangrove tree, then I noticed that the roots were non-flammable.

What I expected to happen was...:
The roots would already be burning and then gone.

What actually happened was...:
The roots were A-OK.

Steps to Reproduce:

1. Get a Flint & Steel, Mangrove Propagule & Bone Meal.

2. Place the Mangrove Propagule anywhere you want.

3. Use the Bone Meal on it and watch it grow into a Mangrove Tree.

4. Once it's a Mangrove Tree, Burn it down and pay close attention to the roots.