Lava flowing onto water acts strangely

Under some circumstances, flowing lava and flowing water do not interact.
How to reproduce: dig a trench 4 blocks long, and place a lava bucket at one end. At the other end, make a hollow 3x3 "pipe" 4 blocks tall (glass is recommended to increase visibility). Place water at the top of the "pipe" and the lava and water shouldn't interact. See the screenshot for details.