so… I was able to find a *very hacky* solution:
(function($) {
acf.add_filter('select2_args', function( args, $select, settings, a ){
var selectId = $select.attr('id');
// this is what we need to bind the event to select2:
var $input = $('input#'+selectId+'-input');
$input.on( 'select2-selected', function (e) {
console.log( 'selected', e );
});
// return
return args;
});
})(jQuery);