Zilean's ult causes the target to become untargetable and still able to at least AA
I was just in a game where a Zilean ult'd his Kalista. When she went untargetable she was still autoattacking and even got 2 kills. I dont know if she wasnt able to move or simply wasnt trying. I dont know how to replicate this bug.
This is the match in question: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1821371470/46796382?tab=overview