The placeable versions of the Pango axes are not real Pangu axes, but actually Pangu Axe Spawners. The actual item does not spawn until you have it as a quest.
I have the quest, I got the key, saw the Axe wasn't there (you see it from another ledge), but went until the end anyway. Pango did not appear, though I do have the original copy in another family. So my theories about this are: 1) no two Pangu axes can exist in the game, 2) Wait 'till it spawns, as you said, 3) cancel and accept the quest again, or 4) game is really borked. Will keep testing until I find out what happened.