On-hit effects and Mordekaiser

So I was building Tank Mordekaiser because I can, and I thought building Titanic Hydra would be a good idea on him.

Not exactly.

The Hydra's passive, active, and on-hit effects in general by extension, do not proc off the boy's Q. Or they do. As far as I can tell, they only proc off his second hit, not the first or third. Is this a bug or a deliberate design choice?

By the way, we ended up winning that game. Even with a random and clumsy item, Mordekaiser still hits like a truck.