Shadowheart also allows the night with Mizora, as well.
Spawn Astarion has never reacted to Mizora at all, it just straight up skipped his dialogue because of, to my understanding, miscommunication in Larian. It wasn't there, but the bug was notified, so they added it in now. The dialogue you are talking about has always been exclusive to AA, it hasn't played for Spawn at any point.
It's also worth noting that I don't think they're demeaning people who choose to romance Halsin- because there's a pretty vital difference between polyamory and cheating. With Halsin, you ask for his consent, with Mizora, you don't. It's a betrayal, and I think that's what Astarion is hurt over there.
