Since the last patch, the triggers for that scene have apparently changed a little - no idea if this is on purpose or a bug. You're right that before the patch it could be triggered simply by entering Moonrise. Apparently, as you've discovered, that no longer works. I recommend continuing to avoid you-know-who - maybe avoiding Moonrise altogether - and instead finishing Astarion's personal quest. That used to trigger the full scene. If it doesn't, keep working on other things in the area which don't trigger the final showdown, and long rest a lot. If you get desperate and there's nothing left to do, then you'll probably just have to go get the "Moonrise version" to make sure the romance stays on track.
