Lava is not spreading fire

The bug

When a lava source is near a flammable block, it should spread fire to that block if in range.
In the below image, the lava should cause the leaves near it to combust. Nothing occurs however, and lava does not create fires.

How to reproduce

1. Ensure that gamerule doFireTick is true
2. Place lava down
3. Place leaves or other flammable blocks near it
4. Wait, or turn up the random tick speed
5. Watch as nothing combusts