That's interesting.  Because it never happened to me in .914, ever.  I have killed entire stacks in .914 and earlier with pillar of flame and the stack would dissappear completely (like it should).

Do you have a link to the .914/other bug report of the same?  I want to see what similarities there are.

Hell, in 914 i was using pillar of flame and tidal wave to control my territory

I was using PoF regularly and successfully in .914 to protect my territory.  Never once had it reduce a unit to 0 hit points, then leave the unit alive. 

However, if you save, quit, and reload the game, the monster with 0 hit points will be gone.

I've seen tidal wave do damage in 0.915 and also seen pillar of flame reduce a troll to 0 hit points without removing it, Sov attacked it and killed it again in tactical combat. Also seen killed champions of opposite alignment stay on the world map, non interactive until a save/reload when they are properly gone.