So if you play Astarion and don't ascend Karlach just dies and none of the other companions try to convince her to go to Avernus?
Unfortunately. The characters have a line-up. Lae'zel leaves first, then Gale considers the Crown and gets taken away by Mystra in his Origin, then Astarion starts to feel the effects of the Sun. I don't know who comes next. If you play any of these characters you get taken out of the scene and Karlach's end sequence plays. She is having a monologue and goes up in flames - even if Wyll is the Blade of Avernus and stands right in front of her.
The patch yesterday changed the order in which Karlach and Mystra's scenes play, so I had hoped they had sussed something out for Astarion as well, unfortunately not. I think Githyanki players choosing to follow Lae'zel also can't save Karlach unless they have a bug, which forgets to take them away from the dock.
I found both Astarion and Gale's dock scenes hard to bear for that reason. Sitting behind boxes while your good friend goes up in flames must be pure horror and coming home from finally being freed off the bomb in your chest to find your lover with severe burning injuries doesn't sound great either.
Edit: I do also have the feeling, that Astarion's epilogue might be a little bugged. I tried three of the possible choices (let's be heroes together, sun protection and moving to a quiet home) but the epilogue assumes an adventuring live-style for all three of them in the narration. I might try the underdark one later and then write a ticket. It would make sense why his conversation is so short if the game assumes you have chosen nothing and thus can't give you any further dialogue.
