EDIT: had same problem, on the 5th try, went thru and focused on killing ONLY the henchmench spawn. Eventually killed AoN, and the cutscene triggered.
This needs patched.
I just had this happen to me so I hope I can help out someone else, when this happened to me though one of my party(Sera) wouldnt heal, she stood back up but still had the red skull instead of health. I too had left one of the little spiders alive and went to kill the boss instead, so maybe this started it. I had Vivienne in my party so I used Resurgence (even though i used it already I managed to build it back up during the fight) and it healed Sera and the cutscene activated. i was relieved too because I was playing on nightmare. So maybe if you want to be on the safe side use vivienne in case this glitch happens to you
