thank you for your help John.
since there is no “out-of-the-box” method, I enacted plan B.
replaced radio buttons with checkboxes and added some JS.
here it is, in case somebody else would ever need it:
(function($){
$(document).ready(function() {
// cache the checkboxes
var
$acftable = $('table.acf-table'),
$checkboxes = $acftable.find('ul.acf-checkbox-list input');
// delegation will make sure click on newly added elements will trigger the function
$acftable.on('click', 'ul.acf-checkbox-list input', function(){
// uncheck all
$checkboxes.prop('checked', false);
// check the clicked one
$(this).prop('checked', true);
});
// if checkboxes exists on the page, but none is checked ... then
if ($checkboxes.length && !$checkboxes.is('checked').length) {
// ... check the first
$checkboxes.eq(0).click();
}
});
})(jQuery);
