Kalista Passiv bug

Today i played a game as Kalista with a friend of mine and as usuall i put my passiv on my support. I've heard that you could change the passivs oathsworn by buying another Dark Spear and using it onto another one, i thought to myself why dont try it at the start of the game, and see, it worked, i could switch my oathsworn. The thing is just that my oathsworn wouldnt deal the W-skill dmg and i couldnt use the Dark Spear another time later aswell. May its intended and may not but it costed me a game without these few % of extra dmg.