Lava that stands by the step filled with water (turned side without water), turns into obsidian (Lava "thinks" that it stands with water)

What I expected to happen was...:
Lava should just remain the head.

What actually happened was...:
Turns into obsidian

Steps to Reproduce:
1. Put a step, fill with water
2. Surround with blocks so that water does not flow

3. Put the lava next to the step, so that the Y axis of the lava is at the same level.