I’m not personally a fan of Astarion, but see, he doesn’t have some of those vampiric attributes because the tadpole is interfering with them for his survival. So it’s the tadpole that’s keeping him from burning in the sun, for example. There’s a plot reason why he doesn’t have some vampiric attributes.
The explanation doesn't really matter to me considering the super tadpole is a complete fabrication of Larian and technically should not work on undead anyway.
What remains is that Astarion is a vampire spawn in name only.
