Oh so after you sleep with astarion for the first few moments after you wake up he's not silently basking in the sun....

he's silently screaming in it

while that would be interesting... I don't think this is intentional and he is meant to make this face. I think this is the random animation when the game is waiting for us to pick a dialogue option.

Their facial expressions don't freeze while they are waiting, they added this for realism. But most of the time they make grimaces which express negative emotion. I don't know why they picked that as the animation that plays most often. But in most scenes these facial expressions make no sense. So basically it's random. (or rather it eventually always turns to a grimace)

The facial expressions are only intentional while they are still talking or something else is happening and the scene is playing out at normal speed. Just moments after that, the random facial animations start to play. And during early access the camera actually showed Astarion's face in this scene and he was smiling even though he wasn't facing the main character. So yeah, this is probably just his face changing as the random animation starts to play.