Oh that thing. No offence, that’s just sequence breaking I am afraid. The deal is in Act 2, you see her again in Act 3, but that’s not something the devs should include or plan for or make it make sense.
Look, this is a very low priority, DE type suggestion on something funny that stood out to me, so don’t take it too seriously.
That being said, when you run into her in Baldur’s Gate for the first time, the first thing she says is, “Hey, it’s you and that vampire spawn! The offer still stands if he’s changed his mind!”, which Astarion obviously rejects. He even responds differently depending on whether or not he’s ascended, so they clearly took his quest into account. That’s why I’m genuinely surprised there wasn’t an option to bite her yourself if that’s what she’s so hell bent on. I’d even have taken an “ew no, I want him”, but acknowledge me damnit!