This is a stealth change with the Skitari release. They were meant only to have reduced impact, now purple enemies are immune to stun from electrocution, such as Arbitrator’s dog with augment, shock maul special or Skitarii kit.
I can clearly tell they weren’t immune to it because on of my go-to builds for Havoc with Garden was Arbitrator with voltaic augment for the mastiff to stun purple Crushers.
They are 300% harder to stagger, practically they got their own stamina bars now. Chorus Zealot can lock them down fine. It feels like they mixed in “rotten armor” as well as crusher patrols are more common with the heal buff.
From what I remember, forced electrocution was not affected by impact resistance. Shock maul special used to stun purple enemies.
It’s no longer the case and now they are fully immune.
Only ults can stagger purple, been like that for a while (bad modifier)