I have seen a couple of clips of Avatar-Karlach and other tall characters kissing Astarion, in which case he has to look up, this gives a good view of both characters faces and also makes this weird leaning/stooping/bendy knee motion unnecessary. It looks nice and more natural, compared to Tavs that have the same size as him. But in both cases the angle also conveniently hides that Astarion's hand isn't animated at all. So, I think it's not a bug but that they ran out of time to make it look nice for all player characters and the same-sized player characters might just got the shortest straw in this case (weird angle, weird movement, stiff hand - stiff hands really as Astarion's left hand is also just flopping about) I hope that was less confusing. ^^
Maybe the expression looks different depending on the model. In my case also both my character and Astarion have what I'd call a pensive expression when separating before Astarion transitions back to smiling when making his theatrical post-kiss comment. It's the one thing I do like about that kiss.
