Water spreads to waterloggable blocks

The bug

It was always stated that water would not spread to waterloggable blocks, unless water was placed inside of them. However this doesn't seem to be the case.

How to reproduce

1. Punch out 3 blocks in the ground
2. Place a waterloggable block, like a fence, in the middle
3. Place 2 water sources in the other 2 spots
   → Notice that the waterloggable block has been waterlogged automatically