Part of the issue is also that it can turn a war that you're supposed to win. You straight up have a larger army, you're winning, but this happens and suddenly your army is decimated (more than, if taken literally). You can write long paragraphs to try and justify it, but it's an extremely annoying interaction that's questionable in it's validity at best.
It can turn war and it feels very wrong yes. But I would argue that it is not as non-sensical when you think about what actually has to be happening on the ground. Even in modern wars, there's a lot of crossfire when a lot of factions are involved, some of them might not even be technically at war. In a medieval world where this type of division does not even exist, you will probably just fight whoever the leader of the army you are attached to orders you to fight.