My gripe is more towards girl character being instantly so chummy and open towards a guy they just know for few minutes, which I've seen way to much lately, I want strangers to act like strangers toward another stranger man not flirting on first day pulling his face close and shit, I check the next and they're basically besties already and they've talk for few min
I also think this when I see a plot where a girl finds a guy who she shares a secret with and seems trustworthy, and then gets physically intimate like that and suggestively says things like "my first". Not that I'm saying it makes this story irredeemable or anything, it's just a trope at the end of the day, but I just want to say that I can relate to what you're saying, I've also thought about it before.
For reference, even with, say, Marin in Sono Bisque Doll having zero awareness of the sexual atmosphere of some of her cosplaying at the start, that's not her flirting, that's her naively thinking "I'm cosplaying with a friend". She doesn't casually conscientiously flirt with him. If all Chika did was get close to his face, that would only be on Marin's level (or the level of Chitanda from Hyouka, for example), but it's beyond that. And I'm not saying flirty characters are unrealistic, but Chika isn't a flirty character with other people, just him.
To sum it up, the author decided they wanted to write a female character who interacts with a boy she's very fond of as a friend by being obviously flirty with him, in a way that's unequivocally flirty, with no explanation, largely as fanservice to the readers and to ramp up the romantic tension, with the couple themselves being kept by the author from ever rationally noticing the fact that this behavior doesn't make sense unless the girl wants to non-verbally show the guy she's romantically interested in him (i.e. the definition of flirting). I'm not saying it's not physically possible for a similar character dynamic to exist, teens are dumb and very often not aware that they're flirting, but it apparently isn't even designed as subconscious flirting, it seems to me like the author is trying to imply that all of these little actions and gestures are coincidences that just end up mimicking flirting accidentally, and that combined with the overall context is why the dynamic feels a bit artificial. It's a bit like no one ever wearing underwear in the manga Saki (sorry if this reference is too obscure) though that doesn't really affect the story at all, just an art style quirk. Edit - Also, the next chapter doesn't have this kind of flirting so I guess it's just the author indulging themselves once. Mostly just Marin-esque naivete.
Also, let me add My Wife is From a Thousand Years Ago and Bokuyaba to the slowburn list.