[Gameplay] Yasuo Ap Bug (possibly sheen/ Lich bane?)

I recently played a game against a Yasuo who built Ap, and did an unrealistic amount of damage. After checking the damage overview after my deaths, i repeatedly saw an un-named section with the icon that usually resembles auto attack damage. This un-named or invisible damage as I'll call it would be about 400-500 magic damage at around level 6-8, around when he got a sheen, this damage continuously showed up when he killed me, and my allies said the same thing showed up for them. He then later in the game used his significant amount of gold to sell his items and build Ad.

How The Bug happened in my game:

1. Yasuo went to lane with an amp tome, got first blood
2. He continued to build Ap, getting a sheen, and got a few more kills
3. He then got Lich bane, increasing the numbers on the "invisible damage" and allies reported the same bug
4. He continued building Ap, doing more un-named damage and gaining a large amount of kills, all while my top and jungle flamed me for feeding
5. He eventually got to the point where his "invisible damage" was no longer significant, and sold all his items to build Ad, having such a lead that he still had the most items in the game
6. He eventually won the game with the lead he had earned, while my top and jungler flamed me and later reported me for feeding