Definitely get the v1.4 patch, if you haven't already.
That said, I've seen this particular bug happen a couple of times even when D3 is patched to v1.4. It's an odd bug, because you should get the cutscene and the end of the level when you kill the Homunculus.
If you have a save point before fighting the Homunculus, I'd recommend trying to go back to that point. Hopefully you don't have to restart the level (although it's one of the better singleplayer levels, I think).
"You seem a decent fellow. I hate to kill you."
"You seem a decent fellow. I hate to die."