I did some testing with the Pestilence and the Heartbreaker gear set and noticed that regardless of the amount of Heartbreaker stacks that you have, the Pestilence DoT doesn't appear to be modified by the 4-set bonus to pulsed targets.
I'm hoping to have confirmation as to whether this is intended behavior or a bug, as the % increase to pulsed targets for Heartbreaker reads as though it should work for the Pestilence DoT, kind of similar to how True Patriot and Eclipse Protocol backpack affect the damage of the Pestilence DoT with their mechanics.
EDIT: Seems like a bug since it was working on PTS.
EDIT 2: This also seems to affect Striker's as well, but I'm not sure whether this was always the case with Striker's or if it was a recent change.