It seems there is no bug with it on Ko'ragh, its just very well design for this fight.
Tried the fight tonight with it and here is the log:
https://www.warcraftlogs.com/reports...e-done&fight=9
If you look at the breakdown plague did most of its damage to the adds not the boss.
So basicly adds spawn and get 15 stack plague on them immediatly and automaticly.
I checked a few of the top mythic kill logs and seems its the same (most damage to adds).
So unless there is some bug with adds taking extra damage from necrotic plague its just a perfect fit for this encounter and nothing more.