Flowing lava doesn't burn flammable blocks sometimes

Flowing lava doesn't seem to burn flammable blocks around it, even with fire spread on.

Easy replication:

1. Build a 3×2×3 wool platform.
2. Destroy the center block on the top (this makes a hole for the lava to fill in).
3. Build a 3 block nonflammable tower.
4. Build a nonflammable platform on the top block of the tower. Make sure it's the same size and same x and z location as the wool platform below it.
5. The center block of the high platform should be destroyed and replaced with lava (using a lava bucket).
6. Watch.

You can also see the attached picture (it's just the same easy replication)