Pestilence behavior in general is kinda janky. I've noticed many different instances where I will have an enemy marked with a large stack and they aren't actively taking any damage. I've also noticed that if enemies die from any source OTHER than my pestilence or a teammates weapons (i.e. My skills, friendly skills, other player's pestilence damage, grenades, environmental dmg, etc) the stack on that enemy simply disappears and doesn't make a jump to a nearby enemy.