Mundo's cleaver is not returning full health cost when killing a minion.

I noticed in a game November 11th on Na server that it would cost 80hp to cast my cleaver, but after I killing a minion with it Mundo would only heal 40hp. The tool tip on Mundo's cleaver reads as "he is returned half of the health cost when he successfully lands a cleaver (increased to the full Health cost on killing blows)"