Single Player, Steam, PC, No Mods.
Demons that are conjured by causing damage with the Demon Lord perk and Zombies raised in the fog of Call of the Dead do not attack the purge insurgents.
How to replicate:
Trigger a purge
Damage the insurgents enough to proc a Demon Lord spawn.
Watch the Spawned demons completely ignore the insurgents.
Equip an Arcane Staff
Cast Call of the Dead.
Watch the zombies wander aimlessly, neither aggroing the insurgents nor making any effort to eat their brains.