[Gameplay] Grinchy Teemo not taking damage from Tryndamere autoattack?

Weird bug. In the linked game, my late-game (URF-fed) Tryndamere was unable to deal damage to the Teemo via autoattacks. Spinning Slash did damage, but Teemo could sit and eat 4-5 autoattacks while shooting me point blank, and his health wouldn't move. No armour items, no lifesteal. I could mow through a Malphite, but not touch the teemo with autos.

Shen was in the game, but this would happen in 1v1s; shield or no shield. I could kill others (quickly) with autoattacks without any problem. My allies did not seem to have a problem killing Teemo (though they did a lot of their dmg through abilities)

For notable examples, see fights at 17:15, 20:00, and 22:50.