Given that stabbing him in the chest with a wooden stick kills him... I'm actually warming up to the idea that Astarion is actually just an elven noble who styles himself as a vampire because he thinks it's cool and edge - he's filed his teeth, tried drinking blood, and it all started because he has an acute allergy to most natural water that hasn't been magically purified... and he's pale and pasty, and burns easy. The only trouble is, he's bought into his own charade so hard, he legitimately believes he's a vampire now... but he's really just... not.
I agree with this fan theory, because once again we have a situation where a character isn't acting at all like what they are supposed to be. Astarion is, in no way, a vampire spawn. He can't walk on walls, no regeneration, no forbiddance, no place of rest (so stake in the heart anywhere destroys him), no sunlight sensitivity, no claws, and he can't bite more than once a day - oh yeah, and he bleeds to death, has no resistance to necrotic, bludgeoning, piercing or slashing non-magical items. Hmmm.
So, how is he a vampire spawn in any way? Oh. Running water hurts him and no reflection.