For what it's worth, there seem to be three triggers which get (a version of) that scene:
1. Talking to the drow in Moonrise
2. Completing Astarion's Act II personal quest
3. Having very high approval with him by some point in Act II (this is sort of a failsafe since it's very possible for the first two not to happen)
Unfortunately these can be messed up by bugs or by not long resting enough.
I was wondering if you also know the answer to something I've been thinking about.

I'm trying to trigger a particular Astarion scene in this route, but I don't know why it's not happening already.
I mean the apology scene, so I'm avoiding talking to the drow for now. I have exceptional approval (full 100, have had since Act 1), have talked to Raphael in Act 2, but not finished that quest, and entered the Moonrise Tower. I was in the impression that the approval and the location would be enough to trigger that scene, but nothing. Might you know what I'm missing?
After entering the tower, I have two other companions whose romance scene triggers in the first and second long rest (Gale and Wyll), but I've told both of them I'm not interested. Naturally I wonder if either of those affect this situation. If I keep resting more nothing happens.