Trying to place lava next to a slab (or upside down slab) or stairs by clicking on the slab/stairs doesn't work. In fact lava cannot be placed by clicking on any face that would result in a waterlogged block is using water.  Other examples: -

• clicking on the block below a pickle - would expect the pickle to break off and be replaced by lava
• clicking on any face of a trapdoor with a lava bucket - would expect lava to be placed adjacent to the face you clicked