I think any D&D campaign heavily featuring religion almost inevitably involves questioning both the mortal religious structure and what the gods in question are actually all about.
One of my players in my 16+ year campaign plays a cleric/paladin (now just a paladin under 5E) and him wrestling with his faith has been a central theme of his character from the beginning. It's been handled in a really mature and sensitive way (since it's a D&D take on a real world religion that he's a member of and I was raised in).
Having the BG3 characters go through this feels like par for the course to me.
It absolutely is par for the course, yeah.
In a lot of campaigns where you've got a heavily religious character they spend a lot of their time either at odds with their faith or exploring what it 'really' means.
Take Dimension 20's Kristen Applebees! She starts out as this fundamentalist member of essentially a religious cult of sun worshippers and slowly, over the course of the multiple seasons, realizes that that particular deity's doctrines aren't for her, eventually rejects him, and seeks out new divine inspiration. Ultimately culminating in essentially creating a deity, whole cloth, from the very fabric of the cosmos. One which holds to her ideals rather than the other way around.
I don't think it's a -bad- thing or anything. It's just an interesting observation that's even driven home by Gale's ascension, in retrospect! One god I didn't include in my overview!
Everything that he was is, essentially, gone. Replaced by a pompous windbag who offers no aid to others or particular rewards because they can earn them through their own ambitions, even though his whole ascension is a big to-do wherein he wants the power to affect -good- in the world and positive change.
Instead he becomes just as detached and disinterested as the gods of Faerun were -before- the Time of Troubles.
... okay so he was a pompous windbag -before- ascension but somehow it's worse and colder afterward.
And Astarion's the same way, even though he doesn't become a "God" with his ascension, just a god among vampires. His selfishness and cruelty are amplified, even to the point of turning Tav into a vampire spawn to serve him, just as he served Cazador, with no compunction about the horrors or pain he's inflicting on them.