For me it was that
even when I knocked out Ravengard in the fight at Gortash's coronation the game registered him as dead. I assume that's because deciding his fate is an essential part of Wyll's quest. If you kill Gortash at his coronation, before Ravengard is imprisoned and in need of rescuing or resurrection, there would be no deal for Mizora to offer Wyll. So if you want to kill Gortash without the quest glitching you either have to kill Ravengard in the fight or wait until after the coronation when he's no longer in Wyrm's Crossing.