Honest question - how are elves supposed to act? I mean tieflings are meant to be evil because of their blood, but we are being told by 5e that they are basically the product of their surroundings, i.e. will become evil if faced with distrust and racism. Wouldn't it be the same thing for elves? I haven't gotten far, but Astarion seems to have as messed up a background as everyone, plus he is a vampire spawn which is the evil creature in itself. If you go back to old games, elves in BG2 Suldanesselar were distrustful of everyone and put geas on Viconia if she was in the party, kicked you out of the city later on and were completely unapologetic for Irenicus. Those were very hypocritical and human things to do.
I take it you have never read a manual. One of the reasons I love Forgotten Realms so much as a setting is that they go in great detail describing what makes every folk unique (particularly so in the older editions). There are many things that set elves apart from other races, both mentally, physically and emotionally. They did a great job with Githyankis, per instance, I just wish they would have been as meticulous with every race. Granted, there are so many races, it's difficult to keep track and spend resources on everyone.
Now I am not asking that every elf behaves the same way, that would be boring. But for a world to make sense, and for fantasy races to make sense, the average NPC should behave in the ways that set them apart from humans, otherwise... What is the point, exactly? And so far, no elves have stuck me particularly elvish. The three biggest elven characters I've encountered show no signs, Halsin and Kagha don't even revere an elven deity.
Also - isn't Shadowheart a half-elf? Anyways, I don't think just because she's half elf means she has to "behave like an elf". If she grew up around humans, she'd certainly behave more like humans do? At least that's what my experience is.
Shadowheart is hands down the best written companion, she became my favourite much to my own surprise. I'm just disappointed that out of the two characters that have elven blood, be it full or mixed, none display an easily recognizable, stereotypical elven trait.