When in offensive stance, after he lands a light and switches guard, if he doesn't attack straight away with the an offensive light, then Highlander drops out of Offensive Stance for a split second. So, if you try to delay your 2nd offensive light after switching guard, you will be throwing normal light that is easy to parry. This is why you have to either spam offensive lights, use emotes or have bigger delay between lights, which means free gb/light for an enemy.

It's frustrating to lose a match due to unintended thrown defensive light or to always keep in mind not to do anything during this auto-cancel for OS.