I've just completed the party night and the Astarion scene triggered perfectly okay for me. Though perhaps I'm not understanding exactly what scenario you're describing when you say you choose it after everyone goes to bed. He suggests meeting up after everyone has gone to bed, but is that all you mean?

If the response is a bit spoilery then you can pop it in tags.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"