Lava not setting wood on fire

See attached screenshot. A flat surface of wood never catches fire from lava placed on top of it. I've left a world open like this for over an hour with nothing catching on fire.

I've confirmed that doFireTick is set to true. If I create a one block hole and put lava in the hole, the surrounding wood will quickly catch fire. If I put a block of wood at the same Y level as the lava, it will eventually catch fire, and then set the rest of the wood on fire.