Lots of quest items and lootables seem to like despawning once you click them once - regardless of whether or not you are actually earning credit for them.

I just found my 5th item for "Finders Keepers", and while I earned credit for the achievement, a mob struck me and interrupted my looting. By the time I went to retrieve it again, it despawned mid-cast bar. From what I've been reading, this happens to just about every Pandaria Treasure, and this has happened to me numerous times on various quests. Especially that quest in Valley of Four Winds that involves burning ropes. If you right click while moving, for instance, you'll get the error that you cannot do that while moving. You won't earn credit for the quest, and the rope will despawn.

The same logic can be applies to what seems to be every single Lost and Found treasure. Though the rope quest is a mere annoyance, this happening with the treasure can lead to LOTS of frustration and disappointment.