Exactly as the title states, using a ranged melee such as Top Tree Dawnblade's Celestial Fire or Middle Tree Stormcaller's Ball Lightning will not proc Warlord's End AoE Effect if it kills the enemy outside of regular melee range.
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Sometimes it may appear that it's not properly working with the ranged projectile melees which might be a bug. But I can confirm that the AoE pulse wave debuff comes from you, the player and not from the enemy you kill. So it's understandable you would think it's not working. You just need to be somewhere within melee range to really notice it depending on the enemy you kill.