Spoils of War not working with Kalista's W's Passive (Sentinel)

I don't know if this is a bug or not, but it highly affects gameplay in a way I took for granted. I played two games as and bot, and what I noticed is that if has a target marked with W's passive, and you try to proc , you kill it, but Kalista takes the minion kill and a charge isn't used.

This lead to some issues, mainly I was money starved and I had no way to regenerate health.