Passing thought: Technically, as a vampire spawn, Astarion should have the spider climb ability. One could explain that as the tadpole repressing it, he notes that it's shut down his regenerative abilities in a banter with Shadowheart.

But if you let him ascend he talks about it in his hubristic rants like it's something new and special. So he doesn't know that he should already be able to do this as a spawn.

One answer is that Larian just decided that spawn don't have that ability in their game, but another answer - fitting DnD canon - is that Cazador didn't tell him about it. Many vampires teach their spawn lies and don't teach them full extent of their abilities to keep them weak and afraid of retaliating to reduce the chances of them overthrowing them.

(I also have a mental image of him discovering this over the events of the game and he has an enraged freak out as he realises he's always had more power than he realised and that Cazador tricked him out of. (Somebody should maybe get this guy a copy of the Libris Mortis. Like, "here's a manual on your condition and similar ones, because we can't trust whatever your shitbag abuser told you."))