Mordekaiser - Mace of Spades + Titanic Hydra

Mordekaiser's Q is unfortunately not working properly with on-hit effects. Having built Tiamat and Titanic Hydra, I noticed that sometimes it would proc on the first hit, sometimes it would proc on the second hit, but never on the third hit (unless I used Titanic Hydra's active ability.) Using Titanic Hydra's active also does not reset his autoattack timer if his Q is active. This behavior has no consistency and is obviously unintended.