Kog'Maw dealing dmg after death- Not the explosion

So far this has happened 3 times, 2 times from me playing kog'maw and recently in an aram i fell prey to this strange bug. For some reason, kog'maw gains the ability to auto attack in melee(?) range after he goes into his purple form but im pretty sure only once because it only happens if they will die from it. Everytime it has happened it has always been a crit, not sure if just randomly or if it has to crit. The reason I know it is not him attacking right before he dies is because it usually happens about ~0.5-1 seconds before he explodes, because it takes him that much time to run up to someone. In the recent aram game (21 games ago) I was playing kassadin and kog'maw died right before me as i was at low health. I riftwalked behind him into the enemy nexus turrets because I needed to back, and he ran up to me and crit me right before exploding and severely damaging an allied volibear. So far I have only observed the crit attack in melee range and it is always a finishing blow, not sure if connected or not.