It's not a bug. Troops going out of control is a bad thing.

But if I am about to kill a specified enemy stack I can take UNLIMITED number of chosen units.
For example, I may hire 1000 knights for all dragon enemies and take nothing else.
My knights will just fight alone and their number and bonuses should do.

I must check what will be the result :)

EDIT: You were right. The game automatically finishes battle when only uncontrolled units remain.
Just when I thought I can attack the whole world with millions of peasants ;)