Does the GY effect of a monster, who was negated by Dracotail Mululu, still resolve?

From what I know about rulings, the effect should still resolve since a card in the graveyard is considered a different card than the one it was on the field. That's why a monster negated by imperm can still have its effects resolve in the grave.

But some people from my locals have said that a Hieratic Seal, negated by mululu would still have its trigger effect that activates when it is tributed, negated. This doesn't make sense to me since a negated Seal could still activate its effect to tribute itself and bounce, the bounce would be negated but he could still tribute as cost. Then after he was tributed his trigger effect would activate and resolve normally since it is a separate effect that activates on grave.

Am I mistaken in my understanding? Because I don't see ant relevant factor here that would make Mululu different from imperm. I know imperm targets and Mululu doesn't, but I don't see how that's relevant.