The Cultist seems to mind his own business in combat
At the end battle on level 3, The Breach, I had a group of 2 cultists, 2 chanders and 4 gnarlings attacking the Lord and the Inhibitor. As soon as I removed the Rally All Minions flag, the Cultists would promptly decide to abandon the fight, perhaps picking on the enemy doors around, but not sticking to their current foe, the Lord or his Inhinitor.
Bohser's response is fairly comprehensive and makes sense. This has always been a difficult area to balance on aggression radii. This is not a bug but we'd always welcome suggestions via the forum on how you might invision improvements without causing further issues.