Aatrox passive won't always proc on-hits

Aatrox's basic attack while his passive -- Deathbringer Stance is up won't always proc on-hit abilities.

I don't know if it's all on-hits, but certain Tiamat and Hydra items, as shown in the clip. I also couldn't tell if it wasn't granting the proper healing from the attack either, since I was at full health.

https://imgur.com/gallery/oeXWZ15

In the same game, I finished Titanic Hydra and found that it, too, would not always proc on the Deathbringer hit, even with the item's active.