A combat seems to be terminated correctly only when all opponents are killed.
When you run away without killing any enemy that is in fight mode with you, it looks like this mode still remains active, even if no fight is ongoing anymore.
You will note this if you approach the opponent again: When you get closer (but not so close to be engaged in combat again), you will still get attack sound and animation from the enemy, as if he is still infight. This will also explain why you get exp when someone else kills the mobster you've been running away from, sometimes even hours later.

Even if this bug does not seem to cause any ingame problems, memory leaks because of this are almost certain. Together with tons of other leaks, they might cause serious resource problems after a while in the game.