Devourer + Harvester of Sorrow

From my understanding, the devourer proc functions as an auto-attack with zero base damage. It doesn't work with Spellblade procs etc since the proc is consumed by the first auto before devourer procs. In the case of Haverster of Sorrow on Morde, it would only be logical for the extra auto to proc the subsequent scaling of Morde's auto but it doesn't. Even with devourer proc, it still requires three true auto attacks to correctly scale. Not sure if a bug or intended. There's absolutely no documentation on Devourer functionality...