Siphoning Attacks doesn't proc Transfer passive

UmbilicalSyllables

So I was just distributing some skill points on my magblade considering I had gained a few levels in my class lines. Transfer is the passive that generates ultimate every time a siphoning ability is cast, and can occur every four seconds. So to test this, while my ultimate was at 476, I cast Siphoning Attacks, and my ultimate remained at 476. This seems problematic and misleading, unless Transfer is only supposed to proc during combat. If that's the case, then I think the tooltip needs to be updated to reflect "while in combat". Otherwise, is this some sort of bug?

ArtOfShred

Only works in combat indeed. Although I agree it could be updated to specify. I believe several other passives similar to this lack mentioning this in the description. I don't think Warden's Savage Beast passive mentions it either.