I looked into the code, the date field does not work with setting $field['default_value']. You need to use this https://www.advancedcustomfields.com/resources/javascript-api/#filters-date_picker_args
I do not know the correct code you need to use here to set the date, you will need to look at the jquery date picker docs here https://api.jqueryui.com/datepicker/
