Kill gain for non-attacking allies

So my wife, my friends, and I have been experiencing this problem where they are either leashing or supporting me/ally and when they leave when the enemy is about a quarter life (25-30% at least) to allow the adc or jungler to gain the kill it instead decides to award the kill to the supp/leashing ally instead. This has happened to happened to other players as well not just me and the thing is they also back off from the enemy.