Item Guardian angel was up , but it did not activate?

As the title says, ITS THE BUG!! I've seen streamers had this bug. Yeah i know its a small bug but that could turn any team fights around. [vayne-pose] I didn't really care when it happened to me since i lost that game. But what if it happens on a comeback team fight that you thought you'll win because you have GA . [sg-soraka]

EDIT: Patch is 8.1