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I believe the same (well, almost the same) happens with LK encounter when he spawns Val’kyr. What happens is that he summons them and they pick up random raider, and if you use stun effect on them before they start movement (they can be targeted and dpsed/cced before they start flying) the Val’kyr will suddenly appear at the end of its path, making the drop instant. It feels very related to me.
No, my problem effect only the player character. NPC (creature) is not effected.