If you tap on the number to add a custom amount, the keyboard on android obscures the form.
This would not be a problem if the app let you scroll the page, because then there would be a small area where you could still see what you were typing, but the entire page is static.
You can’t backspace over the first digit, so if it originally said, “100” and you tap “<” three times, hiding the keyboard reveals that it still says, “1”, so if you wanted to change it to “567” for example, you’d get “1567” instead.
Related to: Add pennies to pots