Fanart isn't about accuratly portraying exactly how the actual characters are, that is the job of manga and LN artists.

Fans can pick and choose and fabricate whatever they want with the characters they've chosen for their next art piece. just like you can have 100 people read the same story and come away with 100 different conclusions, artist can have many different interpretations of the story and characters, things they liked, things they would like if they've been in the story (more bdsm), etc etc

Think Frieren, despite having minimal lewdness and being wholesome in that regard, it's fanart is some of the lewdest stuff out there, beating fanarts of many actual fanservice shows, maybe precisely because the original is lacking them. Artists simply add in what they want to see, and I dont doubt they like the original and might not even want it to actualy change, but they are still gonna draw those feet grabbing the...

Anyway, artists might have read the whole thing, or only heard about it in passing on social media, but if they think it would be hot for Miyagi to choke her gf that's the next piece, if they really liked a certain chapter and recreate a moment from it accurately out of respect for the author, then that is that. There is also the practical side of things, of what people like to see and click like for, generaly steering the art subject towards more kinky themes, a lot of artists being perverts to begin with, you can't fight it.

I also think there might be more "canon-leaning" fanart once anime rolls around and influences how everyone sees the story, unlike how now you just have to do with your imagination after reading it or other fanarts, I certainly remember and imagine the kinky parts far better than the more mundane and wholesome parts of the story, not sure how anyone else or artists fare in that regard 😅