Water flowing from a source block stopped above the last block of a lava flow.

What I expected to happen: the water flowed over the lava flow, the lava flow changed to cobblestone, and the water continued to flow

What actually happened: the water flow stopped above the lava flow.

Steps to duplicate: place water source block at edge above last lava flow block. (first and second image)

Does not occur with second to last lava flow block--when water source was placed one block back the lava flow changed to cobblestone as expected. (third image)