I was playing a game of ARAM earlier. I noticed that Kalista wasn't connected to any players with her Oathsworn and asked her if she had done it yet. She said she had tried it with Morgana, but it went straight on to cooldown, and she tried to do it with the other champions, but again, it went on cooldown straight after trying. You could hear the start of the animation but it would stop. After looking at Morgana's player info, it showed the Oathsworn buff on her, yet it wasn't actually activated at all on her. No physical or animated connection, therefore Kalista could not use her Ulti and making her W passive ineffective.
Haven't seen it do this before but just letting you know!