Water Texture bug

What I expected to happen was...:
That the texture underneath a staircase is not affected if you fill it with water

What actually happened was...:
The staircase suddendly had a water-texture underneath.

Steps to Reproduce:
1. Create a pipe with two staircases facing each other.
2. Fill it with water so that it looks like a pipeline or something of that nature.
3. Look at the texture of the staircase from underneath.