That's happened to me in a few previous saves with the cutscene not firing, before this patch. I wondered if it had something to do with being rested/being able to use the tadpole in conversations. Doing a long rest just before the fight did make the cutscene trigger, but I can't say for sure if that's why it did or not, it was just a thought I had due to it triggering sometimes and not triggering other times. I don't use any mods either.