I don't think the art changed but just that the quality of the chapters are different than before.
As for the vibes that Seojun is more normal now, that's because he's been using the elf's breathing technique before, which made him sparkly/angelic.
But his aura didn't fit Jin Natra so he learned the evil breathing technique to balance his aura, making him more 'normal', kinda like a blank slate. So his vibes are really different from before.
Yeah, the art definitely changed in the recent chapter updates. I noticed the difference after re-reading the latest arc since I forgot what was going on lol. But you're right, the change is there. I'm fine with it though because I still love the story, and the art, unlike some other series, is still easy to look at and done well enough that it doesn't feel disappointing in my opinion. I do miss the old style, but compared to other stories that switch art styles for a new season, this one handled it better than most. You're also right about the MC losing some of his baby-like features. He does look more generic now, which is a common side effect of most art changes. Still, since it wasn’t a super drastic shift, I think this one’s pretty acceptable overall.

Or did the art change? Sorry, I'm just really not used to the change yet;;-.-)
He looks like he lost some baby fat in his face, and the eyes are a bit different too? Idk it just feels like he's an ordinary background character now rather than the main character (I'm basing off vibes)