I could never get the key to drop or find it. Finally I used Sera's "Leaping Shot" to make it to whats behind the door. If you are heading towards the row boat that takes u to the island stop at the beginning of the wooden draw bridge. On the left side u can make your way along the cave steps a little ways, you will see another outcropping across from u on the same side with the broken bridge just past it. It took me a few tries but get as close to edge as u can, turn back to broken bridge and hit leaping shot. If u can land on the rocks you wont be able to climb up but your party will appear on the broken bridge...problem solved 😁