Game Mechanics: a willowisp despawning during fight

[M.J.]

Had this happen with a lesser mummy (it was definitely some variety of mummy) in Misty Thicket. Was fighting near the goblin camp with a rocky area and a static goblin shaman. The mummy was rooted some during the fight and being tanked with a DS, but no idea what is similar between the others fights and mine. Vanished from existence and lost target, just know it spawned super close to the goblin camp up on the rocky zone-wall, spawned right on top of me while medding.