iOS will fill the form field with the numerical value of the OTP. It probably triggers the default ‘submit’ command of the form after that, knowing that there are probably only two form elements within that view: A numerical input for the number, and a submit button (‘Confirm’ or ‘Next’ or similar).
So I’d say you should focus your attention on the submit button of the form (check if you even have a button with a Submit command) and see what the target destination is.