from my testing there is a very small chance of the lil helpers to miss time the final hit of sinners. It usually happens when the lil helper become ready to hit again just as the lights are flashing causing the helper to attack just as the lights stop blinking.
This window seems very small as...
I don't know if this is an intended mechanic or a bug.
When hitting someone with grey talons ult there is a window where if the ult doesn't kill them and you kill the person hit with the ult by other means you still get the spirit stack.
I have not tested if other players getting the kill...