Khan Academy Version 7.0.2 (34973)
iOS 13.7
iPhone X

In the exercise:
Place value tables within 1,000

The section with input boxes below each of the three words:
Hundreds, Tens and Ones

Tapping anywhere on that section, zooms in that section, the sections left side doesn't move. Therefore the section zooms to the right and the third column "Ones" appears outside the right side of the screen.

To be able to focus the "Ones" input box, that section has to be swiped to the left to scroll the "Ones" input box into view.

In this state when the "Ones" input box is tapped, then the keyboard appears, but the section zoomed out.

The keyboard works as expected.

But if one would want to move the cursor in the input box, (in this exercise it only requests one number, but maybe a future exercise will ask for multiple numbers) then any tap anywhere on that section zooms it in again, and the third column appears outside the screens right side again.