
Not sure if this okay, but I was able to read Chapter 1 at https://manhwatop.com/manga/the-detective-of-muiella2/chapter-1/

I feel that Stella's change in chapter 80 is out of her character and does not flow of the rest of the story. . . seems out of place

I'd say that stella character's interactions at the beginning were like, she dealing with nosy strangers that (probably?) wanted to mess with her. It's just after all those events that we get to see stella open up to us as a character. wouldn't call that acting out of place really.
She's a bit of an airhead, which makes her annoying sometimes, but imo it's quite bearable.
This is also called The General's Attachment and I saw this on WebComics and it is up to Chapter 162 and the author listed is Xiao Ming Tai Ji.