Soraka's Passive on Dead Karthus

So I was in an ARAM, playing as Soraka, along with a Karthus on the team. I kept being thrown off by Soraka's sudden bursts of speed toward Karthus after he had died (when he stays "alive" for a few seconds afterwards via his own passive). Obviously, Soraka taking the speedy ambulance to a champion that's already dead isn't very helpful and it almost threw me into the enemy team unnecessarily because I wasn't expecting the speed boost. I haven't noticed it with champions like Sion, so I'm guessing it's how Karthus' passive works unless I've just not been close enough to a dead Sion to notice.

Am I the only one that's noticed this?