Bard and Kog'maw bug

So when i was playing, i was up against a and bot, nothing too bad, but later in the game, me (the support) and our adc went in and killed the but right before he died, ulted kog, and his passive kept going but bards ult hit kog, and kog was even covered in the ult visual effect, but he was still able to move and do the damage, which is a huge bug.