When water is placed above a water source block, and diagonally adjacent to another water source block, it does not spread

What I expected to happen was when water is placed above and diagonally adjacent to water source block blocks, it would spread to fill the air between those blocks.

What actually happened was the water didn't spread and only took up the space in which it was placed

Steps to Reproduce:
1. Dig a 3x3x2 hole
2. Fill the bottom of the hole with water source blocks
3. Place 3 water source blocks above the water on the bottom, in a corner