If I cast Haunt on an enemy and it's ticking along, and then later cast Piranhas on the same enemy (which gives a 15% damage bonus against affected enemies), will Haunt do more damage for the duration of the Piranhas, or does Haunt have to be re-cast to gain the effects of the new damage buff?
Now, I can not say for sure about Haunt and Piranhas, and even if I could, anything could change tomorrow with a hotfix.
What I am getting at, though, is that Blizzard has successively fixed those pairs of skills/effects that "snapshot" damage. The reason for fixes is the opposite of what you describe - some builds are able to field a significant damage/skill bonus for a short while, and damage over time effects applied during that timeframe would "snapshot" the increased damage and continue to deal it for a significant amount if time, even after buffs have fallen off.
I cannot find mentions of skills being fixed in such fashion in hotfix logs since 2.0.1, and the only mention in patch notes is that before 2.1.0 Ancients did snapshot attack speed when summoned. But latest hotfixes mention fixes to legendary item procs snapshotting quite a bit.