Might be some spoilers to the story so I will put my thoughts in tags.

I think it would be a nice thing if they somehow told during the game how that works, for those of us who think about Astarion's character storyflow and eventual usage of netherese magic in curing him or for those of us who think of it just from the game's and world's rules perspective. I'm actually in both camps and really hoped it would have been adressed more during the game. But i don't see this as a huge problem; there is many possible explanations we could do for this to work out in the end(that should be in the game).

First and foremost possible thing is that the netherese(associated with shadows and undeads) magic altered the curse while it affected the brain and body; it was also implied in-game. I don't know if You are acquainted with D&D books, but Van Richten's Guide to Ravenloft introduced playable race for players - dhampir. Despite the funny(for me at least) associations You might have with this word, it gives pretty cool background and hooks for players by making them partialy cursed with the hunger that comes with being undead, be it for blood, flesh or... cerebral spinal fluid. Along with it You are getting a "vampiric bite"(but You can still fluff it all the way You want as something other visualy), You don't have to breath(because of your now deathless nature) and have darkvision if You didn't had it before; plus bonus in the climbing ability. You also retain something from your original racial feats and don't have to fear sunburns, but don't get vampiric resistances.

As game master I would have used this and said that the tadpole influenced with that kind of magic changed his functional race to a dhampir and when he lost the tadpole, well, the curse returned with all it's might. That would be the simplest explanation for the game rules I think. The partial ceremorphosis choice would also have been cooler if it had the consequence of hunger for that cerebral spinal fluid.

What I don't understand is why they made other vampire spawns visualy different and why they gave them those weird tiefling's eyes. I know they wanted to make Astarion special and them more monstrous maybe, but for me that looked bad and didn't go well with the storyflow. Imagine how hard it is to explain to every inn's guard or victim that no, You don't have weird monstrous eyes and You can be trusted. No, vampires are scary because they can mingle in society so they should be able to do it and that's probably hard with those eyes. Even fantasy worlds have some established rules; they could at least get "normal" vampire eyes after Cazador's plan fiasco. That would have been a good indicator of his influence being gone from them that players not acquainted with D&D could cleary see. But that is just my personal complaint.