I noticed Mass Hysteria won't have a visual effect if all potential targets in range of the spell have CC immunity, meaning it doesn't hinder gameplay and is simply a visual bug.
Four videos of Mass Hysteria wrong working
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Four videos of Mass Hysteria wrong working
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the white swirls around the NPC's feet mean they are currently imune to CC effects.
this is caused because you cc'd them earler (e.g. with ambush) and then there is a 5sec interval (after they're back on their feet) where they cannot be cc'd again.