This is actually the problem described in the issue you linked in the initial post. The problem is that only a native form submit (with page load) will include the value of the clicked submit button/input. As soon as you prevent the native handling to submit the form using javascript/formdata this won‘t happen, because it‘s no longer aware of which button was clicked. You‘d need to work around that with even more js, to e.g. fill a hidden field with the value of the submit button when it‘s clicked, so later the formdata obj. does include the value of the hidden field.
