Astarion was a slave and a dress up doll for his vampire living in a fancy mansion, then was snatched in the middle of it all by the mindflayer. He should be drenched in fluid, but he should not have had chance to change into suitable traveler wear, that's on you to do.
Well... no that’s on the writers. You’re just restating the story they came up with to justify introducing a character in an outfit that wrecks the story. You’re missing the point.
We could argue about the approximate time of day this happened and why vampires would or wouldn’t be awake but it doesn’t matter. There’s is no “actually”... it’s a story. A story someone created. A story that can be good or bad. There is no series of facts.
The characters are too loud and obvious. The art style is to fantastical and undermines the writing and the gameplay. There’s no ‘Well actually, in 5E grease isn’t flammable’ argument that’s going to excuse the disconnect between our eyes and the theme the creators are going for.