Astarion is the worst companion of them all as he is completely lore breaking. A Vampire Spawn without any of the characteristics and powers he should have and also without most of his weaknesses. And just saying "its the tadpole" is a very weak excuse.
Not to mention a vampire spawn isn’t free until their master dies. But again, cuz tadpole.
I think the inconsistencies are there because they didn’t know how to use him in the game. There’s no day/night cycle so the tadpole magically allows him to walk around during the day. The game requires exploration so the tadpole magically allows him to enter other people’s homes.
But for some reason he can’t cross running water because the tadpole can’t magically alter that... My guess is the writers wanted to remind players and not let them forget that, yes Astarion is a vampire.