Spawn Astarion (as in, not ascended) doesn't have much to say right now if you cheat on him with Mizora in Baldur's Gate 3. But dataminers have found unused dialogue in the files where he gets so upset that he tries to break up with you.
When Astarion realises what you've done, you can plead your case by saying that it was a "mistake", egging him on by arguing that he would've "jumped at the chance" too. But Astarion rejects that sentiment, saying he believed you "were better than other people".
I see everything I've been trying to escape in you. Even I deserve better than this.
You can respond by saying that "it didn't mean anything," but Astarion drives the stake deeper into your heart, asking if your relationship means anything, either. "I thought I could trust you, which was no small thing. Now I'm not sure what to think."
Luckily, you're given a chance to salvage the relationship by rolling a charisma check. If you succeed, Astarion agrees to move on and put this behind him, but he demands you not do it again.
If you fail… "You can promise me the heavens, it doesn't change the choice you already made," Astarion says. "It was selfish. It was you not thinking about anyone but yourself. And that's a feeling I know all too well. I may not have the moral high ground when it comes to betraying trust, but right now, I see everything I've been trying to escape in you. Even I deserve better than this. We're done."
This isn't in the game right now, but Larian Studios may be working to add it in a future update. Time will tell. At any rate, just reading Astarion's hurt at being cheated on is upsetting enough, never mind how gutwrenching it will be if Neil Newbon brings it to life.
There's also new dialogue if you offer to sleep with Haarlep to enlist aid in your fight against Raphael. Before, companions would stay strangely silent, but during the scene as of Patch 6, Astarion warns you against it, saying that you can't trust him.
One of the big complaints many fans have with Act 3 is that your companions don't react enough to the story, especially with scenes like Mizora and the Emperor's romance, but it looks like Larian is working to amend that. And by amend, I mean ripping our hearts out of our chests.
