Why don't kill moves ever trigger when I finish an enemy my follower is currently fighting?

I have my kill move chance cranked at 100% and removed last opponent and perk requirements... kill moves used to occur very frequently for me, but since I picked up a follower kill moves never trigger against an opponent if my follower is fighting that same enemy. Seems the only trigger if it's a one on one battle vs me and an opponent where my follower isn't involved.