I've had the same thing with a recent Yamato Game. Match ID: 31633161 at 30:05 I start a fight and get 2 kills within the same ult and still no skin. Idk if devs can give skins retroactively, but we can hope.
I think it’s a bug. If you watch the replay you need to check if Yamato is still red when you get the second kill. I'm sure she has already transitioned to her normal Color.