Karthus Buff Bug

In one of my team builder games, I played against a Karthus mid. The noticed bug was that he had Red buff on him (pretty much entire game, excluding beginning/early phase.) I'm not sure whether he picked it up from one of my other teammates or from jungle camp but every time we killed him, he came back to lane with the red buff. It may have to do with the buffs transferring back to himself when he dies (since now it still does if jungle minions kill you). Since he "dies" to himself (passive), the buffs currently work to his favor. He never picked up a blue buff so I cant be certain if there is a problem with that one but I'd assume they run on the same principle that it would still transfer back to himself.