Yeah I'm having this exact same issue.
Because I'm a very stubborn person I've spent waaaaay to long on it and might have a solution (which is still not exactly 100% foolproof I guess).
Seems like your approval with him must be just right for that scene to trigger (somewhere in medium-high range) and I might be wrong but you shouldn't progress too far into the main story - for some reason I can't get it on one of my saves where I've already spoken to Kahga and Nettie).
Also I think it's connected with is death quest since I've never really gotten it to trigger without - but the death quest itself has a few ways to bug out on you soooo...
Astarion's stars dialogue is just as whimsical and so is Shadowheart's content.
Al ofl these early scenes also require an ungodly amount of camping - something you'd probably never do so early on
