So I was playing a game this evening as Malzahar when I noticed that an enemy Gangplank was still walking while I hat ulted him. I unfortunately don't have the replay because replay.gg refuses to cooperate, but I'll try to reproduce it in another game. I don't know how to reproduce it however.
Are you sure he didn't just eat oranges and be K? GP is one of the hardest champs for Malz to deal with because he has a free cleanse on his W.
Did 
press W? If you saw him heal and walk away while taking damage that's what happened.
GP is still on free rotation so just get a friend to W your ult in a custom game.
This means he used an orange it is reproduced
if you orange while Malz is ulting.
Lvl 26
definitely ate an orange with his W skill, Remove Scurvy, which removes ALL cc's on him and heals him. Malzahar's ult IS a double edged CC, which does get removed with oranges.
