Tibbers lingers at 1 hp [Gameplay]

On the Howling Abyss map playing as kog maw, could not auto attack tibbers to death. It would display 1 hp and after taking an auto attack would remain at 1 hp. In between attacks it would also continue to regenerate health. It would take several auto attacks with its hp hovering between 1-6 health before it dies.

Details I can remember while this occured: early game (1 rank in ult), w was active (kog), tibbers was partially in bush or standing amongst ally minions.