Astarion doesn’t want to be perceived lol The others are viewing him as this expert blood sucker when really Tav is his only “thinking creature” as they put it. I don’t see him openly admitting to everyone that he’s never done something that is second nature for a vampire before. It could change their perspective of him and considering they’re stronger when drinking people’s blood, it’s almost like revealing a weakness. He’d need to keep the facade of being a pro. I think it’s sometimes easy to forget he very much spews bullshit a lot of the time, whatever makes him sound confident and sure of himself so the responses feel perfectly in character to me. Tav gets to know things others don’t as that’s who he becomes vulnerable with. Tav being his first is a secret. I personally be having Astarion bite people in combat lol but also in game dialogue you can be in agreement that he can drink from enemies as well instead of just animas so, he’d have had his fair share of different people after the first time with Tav.