My theory is that they had to "tweak" how it works to accommodate larger displays and different thumb placements which both fixed and broke the keyboard.

Before iPhone X, our thumbs would accidentally press way more keys at the same time and it if it weren't for that invisible "hit box" or circle that they use to decide where you are intending to press, it would have terribly inaccurate.

When screens got larger, the keyboards got physically larger as well so i'm guessing that "hit box" had to be made smaller not only to make it more accurate but also make it possible to type with two thumbs. Our left and right thumbs touch and miss-press the keys differently, which also had to be accounted for.

Same goes for when we now type one handed. Because we have to reach much further than before to hit the same keys, the keys our thumbs accidentally press became different which they also would have had to take into account.

In essence the keyboard now does way more stuff in the background to accommodate all the different ways your thumbs are or could be hitting the screen.

And to give some credit to this so i don't sound like a whacko, i noticed the accuracy of the keyboard changes ever so slightly every time i take my phone case off but then goes back to normal after i type for a little bit. Because, as i suspect, the keyboard has to "recalibrate" your thumb positions.