[Letharos]

Afaik it works like this:

Did debuff give message? Then yes. That means spell landed on mob, hence aggro should generate. The detrimental part is separate from the buff part.

Tbh tho I have never needed shadow vortex to keep aggro so I have never bothered to check. Are you pulling with it? Or DC for that matter? Because that might explain it. If you're pulling with either one you should consider either manual (bow etc) or maybe siphon strength for pulling.

Edit: to clarify: initial aggro is capped so if you pull with shadow vortex you are gimping its aggro generation. The detrimental part will land even if the buff doesn't stick and that is generating the majority of hate from the spell. Mebbe clarify? Whiskey after midnight etc