I don’t think it’s an indication of anything troubling. Sometimes you just get lost in what you’re reading and momentarily forget what device you’re using. If anything, I’m glad teens are so absorbed by reading. It was a struggle to get the teens of my generation to read anything!
I remember how when I was a teen and coding a lot inside the Turbo C IDE, it became second nature to press F2 frequently (to save).
Around that time, while working on math homework (on paper), I'd occasionally experience brain farts, where I would think of pressing (not reach for) the F2 key.
FWIW, I have decent handwriting today, and even exchange correspondence with some friends using pen-and-paper.
I do agree that touchscreen usage should be restricted for toddlers.
I’ve also caught myself yearning for Cmd-F when hunting for something in large chunks of printed text.
We have some older machines for getting train tickets and some newer ones. They probably didn't want to scare anybody off, so the interface still looks almost the same. It used to be buttons next to the screen, now it's a touch screen.
Of course you see people trying to touch the old ones. There's no signifier on the new ones (except the lack of buttons) that shows that they're different, so without a bit of thinking you don't know which affordances there are.
So the on-screen text says "touch here" or similar. Took me a while to realise it meant "press inside the screen-printed rectangle in the panel below the screen" ...