I tested this, and if I have slide-to-type disabled, and slide my fingers, then every letter I slide over will highlight, but only the letter I let my finger up on will show up in the text input box.

If I don't have slide-to-type enabled, then only the letter I press down on will highlight, and what shows up in the text input box is pretty inconsistent for horizontally adjacent letters.

You can indeed see this behavior in action by tapping a letter and briefly swiping slightly to an adjacent letter. Maybe I’m missing something but I don’t see why the letter should be submitted on touch-up, since I’ve never intentionally tried to correct an individual letter between touch-down and touch-up.