I'm running a 1.9 linux server, just for me and a couple of friends. The world I'm using was copied from a lanworld I had running from the MC client.

When entering the nether, which was generated by the server, I entered normally. I started mining and, as you do in the nether, encountered a lava pocket in the terrain. What I expected was it to flow out... like lava. What I didn't expect was for it to flow out at the speed of water, and travel for 8 blocks.

Steps to Reproduce:
(After reproducing in singleplayer 1.9)
1. Generate a 1.9 world
2. Enter the nether
3. Place lava or find any naturally generated lava
Overworld isn't affected.