I am in beta so that could be it, however naturally generating lava seems to damage you a block away from where it is. I've only experienced it with naturally generating lava. Here is a picture of where it damages me (i am above the lava, and it acts like the block is a flowing lava block as it makes me slow when in the invisible lava) i have not tested this with lava source blocks though I will suspect that it probably effects it too