Scyorielaphas won't yield in battle

I hope this is the right forum for this. I'm at the last part of the quest "The Shadow Under Neketaka". I've seen in walkthroughs and on video that you should be able to fight the dragon Scyorielaphas to submission, at which point you can choose to siphon a portion of his soul without the metaphysics check. However, every time I fight him he just dies without any dialogue. Was this behaviour patched out, or am I doing something wrong? I've tried engaging him after each phase of dialogue, and even force attacking him when I'm looking at the wards. Same result every time.

I found this video and tried to replicate it. It just doesn't happen this way for me. The dragon never stops to say anything when near death - he just dies. I do notice that he tends to take a chunk of damage when he's near death that doesn't look like my usual DPS. The only difference I see is this guy manages to get him down before he casts that tidal wave attack which breaks the wards, while I don't. Is that the issue? A DPS check? I'd like to know before I commit to trying to beat it.