Sated Devourer + Ekko's passive

So, I just tested it out in a bot game. You can check my match history, ignore the crappy on-hit build, please.

But, the one thing I was testing, was Ekko and Devourer. The idea struck me because Vayne procs Silver Bolts every other attack, rather than every three. Vi does the same thing, and Caitlyn got her headshot stacks faster.

So imagine my surprise when it still took Ekko three hits to proc his passive.

Can someone explain to me why? Is it not an on-hit for whatever reason? Is this intended, or some weird glitch due to Riot's coding?