I am becoming a firm believer that Astarion was raised/lived in Waterdeep before ever becoming a Magistrate in Baldur's Gate and being turned.

The fact that his tombstone uses Waterdeep's calendar feels like a hint towards it. Could have been an oversight from the team (it is kind of needlessly complicated) but it would be neat if it was on purpose.

Also, his first answer to the magic mirror below the blighted village hints that he misses his "real home, the one he hasn't seen in centuries." This could be a literal house, but alternatively if we interpret it as a "hometown" instead, it would mean he's not from Baldur's Gate at all.

He certainly fits the bill for a high elf who hails from Waterdeep, imo (snobby lmao) but Waterdhavians are also known to be, uh, not quite as shitty. He might have moved to Baldur's Gate because he knew he could be more successful (and corrupt) there.

Mostly I love the idea that he's from Waterdeep because it adds so much depth to his petty dislike of Gale. Of COURSE he hates Gale at first!! He is the poster boy of Waterdeep and everything a Waterdhavian should be! It's something Astarion would never be able to return to being, forever changed by centuries of slavery and torture. And it makes him bitter, because Gale reminds him of the place he used to call home.