When an ItemStack is assigned to a PlayerBucketEmptyEvent via setItemStack(ItemStack) the ItemStack can be recalled with PlayerBucketEmptyEvent.getItemStack() but when the method is done and the event result is applied it ignores whatever item is set and just puts an empty bucket in the player's hand. You can try this with any code you like, it won't work under any circumstances.

For convenience's sake I'll just attach my own class where the error happened. I have a workaround by cancelling the event and then simulate the event by hand placing the water and putting the item in the hand, but this should of course not be necessary and most likely can cause bugs in certain circumstances.