I’ve seen this in PC reports so tagging with Xbox since I’m on a Series X.
During Chasing Lies, I died to Belial. Playing on Hard difficulty. Upon re-entering the boss area, all the scripted events occur, but Belial doesn’t spawn. Had this happen on another character as well, and had to exit the game and load back in, which resets the entire quest back to the Speak with Sayeena at her Shop step.