With the help of some players, I was able to finally fix this bug in the game. For some reason that I still don't know, sometimes killing a zombie with the rifle doesn't decrease all the enemy health, but the enemy is deactivated and his map marker is gone. Now I just check how many map markers are active on the map. Based on that, I count how many enemies are remaining and consider the level completed when there are no map markers. Please, allow Steam to update the app and tell me if this fixes the bug.