Kalista's Sentinel passive not functioning after rebinding.

During a ranked game, I noticed that after first binding to my support (A Janna) then rebuying the item and rebinding to our jungler (A Gragas), the passive W damage wasn't procing. while attacking gromp, or any other target.

I tested this in a custom game with different champions, (Binding first to an Aatrox, then to an Alistar) I found it to be repeatable. I found that if Kalista hits the target first, it shows the sentinel mark, but neither the first oathsworn or the second can proc it. If the first or second oathsworn attack the target first, no mark appears, and Kalista cannot proc it.

Rebinding to the same partner, however, does not cause this problem.