The bug

Depending on the water/lava formation placed at y=0, the liquid may or may not spread. It would seem like the liquid requires three adjacent source blocks, but I have yet to fully confirm this.

How to reproduce

1. Place lava/water in a "T" shape, and break a block to make a cross
   →  Liquid flows
2. Place lava/water in an "I" shape, and break a block to make a "T" shape
   →  Liquid doesn't flow