Fix the test, which was not actually reproducing the issue
in order to do so, I had to fix the behavior of acs::test::xpath::get_form_values: we should in fact NOT extract the value of disabled formfield, as a real browser will not send them in the POST request.
extend date formfield test to include also a date in a specific format from downstream, set to disabled. Set the day of the month as a number < 10. Show that in this case the validation will complain about an invalid value. This because the field is represented as " 1" rather than the expected "1"