The hunter might not recognize him because he meets Astarion during the day. Vampire spawn are not supposed to be able to walk in daylight so that makes sense. The hunter is not aware that Astarion has a tadpole making him non-allergic to sunlight.
ps. I prefer no redemption arc for Astarion. Some characters should just remain irredeemable.
Though I wholeheartedly agree with some characters should not have a redemption arc,
especially for players who want to keep an evil play through
I feel like it would be nice to have an option on whether to help redeem a character or keep them "corrupted"